Add testing section of docs. Consolidated trybot/buildbot section

BUG=skia:

Preview link: http://skiadocs.com:8000/dev/testing/?cl=954523004

Review URL: https://codereview.chromium.org/954523004

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+Correctness Testing
+===================
+
+Skia correctness testing is primarily served by a tool named DM.
+This is a quickstart to building and running DM.
+
+~~~
+$ ./gyp_skia
+$ ninja -C out/Debug dm
+$ out/Debug/dm -v -w dm_output
+~~~
+
+When you run this, you may notice your CPU peg to 100% for a while, then taper
+off to 1 or 2 active cores as the run finishes.  This is intentional.  DM is
+very multithreaded, but some of the work, particularly GPU-backed work, is
+still forced to run on a single thread.  You can use `--threads N` to limit DM to
+N threads if you like.  This can sometimes be helpful on machines that have
+relatively more CPU available than RAM.
+
+As DM runs, you ought to see a giant spew of output that looks something like this.
+~~~
+Skipping nonrendering: Don't understand 'nonrendering'.
+Skipping angle: Don't understand 'angle'.
+Skipping nvprmsaa4: Could not create a surface.
+492 srcs * 3 sinks + 382 tests == 1858 tasks
+
+(  25MB  1857) 1.36ms   8888 image mandrill_132x132_12x12.astc-5-subsets
+(  25MB  1856) 1.41ms   8888 image mandrill_132x132_6x6.astc-5-subsets
+(  25MB  1855) 1.35ms   8888 image mandrill_132x130_6x5.astc-5-subsets
+(  25MB  1854) 1.41ms   8888 image mandrill_132x130_12x10.astc-5-subsets
+(  25MB  1853) 151µs    8888 image mandrill_130x132_10x6.astc-5-subsets
+(  25MB  1852) 154µs    8888 image mandrill_130x130_5x5.astc-5-subsets
+                                  ...
+( 748MB     5) 9.43ms   unit test GLInterfaceValidation
+( 748MB     4) 30.3ms   unit test HalfFloatTextureTest
+( 748MB     3) 31.2ms   unit test FloatingPointTextureTest
+( 748MB     2) 32.9ms   unit test DeferredCanvas_GPU
+( 748MB     1) 49.4ms   unit test ClipCache
+( 748MB     0) 37.2ms   unit test Blur
+~~~
+Do not panic.
+
+As you become more familiar with DM, this spew may be a bit annoying. If you
+remove -v from the command line, DM will spin its progress on a single line
+rather than print a new line for each status update.
+
+Don't worry about the "Skipping something: Here's why." lines at startup.  DM
+supports many test configurations, which are not all appropriate for all
+machines.  These lines are a sort of FYI, mostly in case DM can't run some
+configuration you might be expecting it to run.
+
+The next line is an overview of the work DM is about to do.
+~~~
+492 srcs * 3 sinks + 382 tests == 1858 tasks
+~~~
+
+DM has found 382 unit tests (code linked in from tests/), and 492 other drawing
+sources.  These drawing sources may be GM integration tests (code linked in
+from gm/), image files (from `--images`, which defaults to "resources") or .skp
+files (from `--skps`, which defaults to "skps").  You can control the types of
+sources DM will use with `--src` (default, "tests gm image skp").
+
+DM has found 3 usable ways to draw those 492 sources.  This is controlled by
+`--config`, which today defaults to "565 8888 gpu nonrendering angle nvprmsaa4".
+DM has skipped nonrendering, angle, and nvprmssa4, leaving three usable configs:
+565, 8888, and gpu.  These three name different ways to draw using Skia:
+
+  -    565:  draw using the software backend into a 16-bit RGB bitmap
+  -    8888: draw using the software backend into a 32-bit RGBA bitmap
+  -    gpu:  draw using the GPU backend (Ganesh) into a 32-bit RGBA bitmap
+
+Sometimes DM calls these configs, sometimes sinks.  Sorry.  There are many
+possible configs but generally we pay most attention to 8888 and gpu.
+
+DM always tries to draw all sources into all sinks, which is why we multiply
+492 by 3.  The unit tests don't really fit into this source-sink model, so they
+stand alone.  A couple thousand tasks is pretty normal.  Let's look at the
+status line for one of those tasks.
+~~~
+(  25MB  1857) 1.36ms   8888 image mandrill_132x132_12x12.astc-5-subsets
+~~~
+
+This status line tells us several things.
+
+First, it tells us that at the time we wrote the status line, the maximum
+amount of memory DM had ever used was 25MB.  Note this is a high water mark,
+not the current memory usage.  This is mostly useful for us to track on our
+buildbots, some of which run perilously close to the system memory limit.
+
+Next, the status line tells us that there are 1857 unfinished tasks, either
+currently running or waiting to run.  We generally run one task per hardware
+thread available, so on a typical laptop there are probably 4 or 8 running at
+once.  Sometimes the counts appear to show up out of order, particularly at DM
+startup; it's harmless, and doesn't affect the correctness of the run.
+
+Next, we see this task took 1.36 milliseconds to run.  Generally, the precision
+of this timer is around 1 microsecond.  The time is purely there for
+informational purposes, to make it easier for us to find slow tests.
+
+Finally we see the configuration and name of the test we ran.  We drew the test
+"mandrill_132x132_12x12.astc-5-subsets", which is an "image" source, into an
+"8888" sink.
+
+When DM finishes running, you should find a directory with file named dm.json,
+and some nested directories filled with lots of images.
+~~~
+$ ls dm_output
+565     8888    dm.json gpu
+
+$ find dm_output -name '*.png'
+dm_output/565/gm/3x3bitmaprect.png
+dm_output/565/gm/aaclip.png
+dm_output/565/gm/aarectmodes.png
+dm_output/565/gm/alphagradients.png
+dm_output/565/gm/arcofzorro.png
+dm_output/565/gm/arithmode.png
+dm_output/565/gm/astcbitmap.png
+dm_output/565/gm/bezier_conic_effects.png
+dm_output/565/gm/bezier_cubic_effects.png
+dm_output/565/gm/bezier_quad_effects.png
+                ...
+~~~
+
+The directories are nested first by sink type (`--config`), then by source type (`--src`).
+The image from the task we just looked at, "8888 image mandrill_132x132_12x12.astc-5-subsets",
+can be found at dm_output/8888/image/mandrill_132x132_12x12.astc-5-subsets.png.
+
+dm.json is used by our automated testing system, so you can ignore it if you
+like.  It contains a listing of each test run and a checksum of the image
+generated for that run.  (Boring technical detail: it is not a checksum of the
+.png file, but rather a checksum of the raw pixels used to create that .png.)
+
+Unit tests don't generally output anything but a status update when they pass.
+If a test fails, DM will print out its assertion failures, both at the time
+they happen and then again all together after everything is done running.
+These failures are also included in the dm.json file.
+
+DM has a simple facility to compare against the results of a previous run:
+~~~
+$ ./gyp_skia
+$ ninja -C out/Debug dm
+$ out/Debug/dm -w good
+
+   (do some work)
+
+$ ./gyp_skia
+$ ninja -C out/Debug dm
+$ out/Debug/dm -r good -w bad
+~~~
+When using `-r`, DM will display a failure for any test that didn't produce the
+same image as the `good` run.
+
+For anything fancier, I suggest using skdiff:
+~~~
+$ ./gyp_skia
+$ ninja -C out/Debug dm
+$ out/Debug/dm -w good
+
+   (do some work)
+
+$ ./gyp_skia
+$ ninja -C out/Debug dm
+$ out/Debug/dm -w bad
+
+$ ninja -C out/Debug skdiff
+$ mkdir diff
+$ out/Debug/skdiff good bad diff
+
+  (open diff/index.html in your web browser)
+~~~
+
+That's the basics of DM.  DM supports many other modes and flags.  Here are a
+few examples you might find handy.
+~~~
+$ out/Debug/dm --help        # Print all flags, their defaults, and a brief explanation of each.
+$ out/Debug/dm --src tests   # Run only unit tests.
+$ out/Debug/dm --nocpu       # Test only GPU-backed work.
+$ out/Debug/dm --nogpu       # Test only CPU-backed work.
+$ out/Debug/dm --match blur  # Run only work with "blur" in its name.
+$ out/Debug/dm --dryRun      # Don't really do anything, just print out what we'd do.
+~~~