test: Uniformly canonicalize actual and expected JSON

Previously, we used a variety of ad-hoc canonicalizations for JSON
output in the test suite, but were ultimately very sensitive to JSON
irrelevancies such as whitespace.  This introduces a new test
comparison function, test_expect_equal_json, that first pretty-prints
*both* the actual and expected JSON and the compares the result.

The current implementation of this simply uses Python's json.tool to
perform pretty-printing (with a fallback to the identity function if
parsing fails).  However, since the interface it introduces is
semantically high-level, we could swap in other mechanisms in the
future, such as another pretty-printer or something that does not
re-order object keys (if we decide that we care about that).

In general, this patch does not remove the existing ad-hoc
canonicalization because it does no harm.  We do have to remove the
newline-after-comma rule from notmuch_json_show_sanitize and
filter_show_json because it results in invalid JSON that cannot be
pretty-printed.

Most of this patch simply replaces test_expect_equal and
test_expect_equal_file with test_expect_equal_json.  It changes the
expected JSON in a few places where sanitizers had placed newlines
after commas inside strings.

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