Improve fragment shader checks in subgroupBitwise

[alan-baker]

• Modifies the fragment tests in subgroupBitwise to avoid any subgroup in the last row or column
  • this fixes failures seen on some devices performing subgroupXor when helpers are in bounds, but not on the edge of the framebuffer
• Tests skip cases that could not possibly yield useful results, but checking is also terminated early if no useful results can be found
  • Enough framebuffer variants are swept to ensure some tests will cover useful subgroups, but the sizes are only known after a pipeline compile

This fixes the errors I was seeing in small framebuffer sizes on Apple silicon.

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 in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

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