Turn on google-readability-avoid-underscore-in-googletest-name clang-tidy warning and fix violations.

https://google.github.io/googletest/faq.html#why-should-test-suite-names-and-test-names-not-contain-underscore

Example:

/b/s/w/ir/cache/builder/src/flutter/shell/platform/embedder/tests/embedder_unittests.cc:1641:1: error: avoid using "_" in test name "SoftwareRenderingPixelFormats_draw_solid_red_kRGB565" according to Googletest FAQ [google-readability-avoid-underscore-in-googletest-name,-warnings-as-errors]
SW_PIXFMT_TEST_F(draw_solid_red, kRGB565, (uint16_t)0xF800);
^
/b/s/w/ir/cache/builder/src/flutter/shell/platform/embedder/tests/embedder_unittests.cc:1634:10: note: expanded from macro 'SW_PIXFMT_TEST_F'
         SoftwareRenderingPixelFormats_##dart_entrypoint##_##pixfmt) {    \
         ^
note: expanded from here

https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8797288801511583249/+/u/test:_lint_host_debug/stdout

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I updated/added relevant documentation (doc comments with ///).
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