sample_mask input: allow either full coverage mask or (1<<sample_index)

[dneto0]

This allows the varying behaviour observed on different platforms.

Generalize the "findFailedPixels" utility to allow two reference "expected" texel views. A pixel comparison succeeds if the "actual" texel matches the texel from either of the "expected" or the "alternate expected" texel views.

See gpuweb/gpuweb#5457

Issue: #

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at c6a5949):

 Test case/subcase counts did not change.