fix race: this kCacheSize is probably not meant to be static

TSAN sees us racing to intialize the static const kCacheSize.

This static const value depends on the runtime value image->getSize(), so there
is a race to set it.  I think this is unintentionally copy-paste from the other
tests that use a constant kCacheSize.  It's weird to intialize a constant based
on the first call to test_internal_purge(), though luckily we're always calling
it with same sized images today.

See these TSAN failures:
    https://uberchromegw.corp.google.com/i/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN/builds/4937
    https://uberchromegw.corp.google.com/i/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN/builds/4940/steps/dm/logs/stdio

TBR=robertphillips@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1720743002

Review URL: https://codereview.chromium.org/1720743002

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