Fragment shader tests for subgroup add and mul

[alan-baker]

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.

[dneto0]

LGTM for testing subgroup add and mul for u32.

Please add TODOs for the other concrete scalar and vector types, then we can land this one. I'm ok delaying writing the remaining cases.

[alan-baker]

LGTM for testing subgroup add and mul for u32.

Please add TODOs for the other concrete scalar and vector types, then we can land this one. I'm ok delaying writing the remaining cases.

The data types are tested in other cases. I don't think there's a ton of value in doing a full cross product for types and stages.