Compat: Add tests for maxStorage(Buffer/Texture)sIn(Vertex/Fragment)Stage

[greggman]

Note: I couldn't really think of a test on the core side. Originally I was going to try to request maxXXXInYYYStage less than maxXXXPerShaderStage which is what the tests do in compat but in core I believe it will be a no-op and that InStage limits will always match their PerStage counterparts.

I added a RequiredLimitsTestMixin to try to make it easier to write a test that needs specific limits. The issue is the key. I thought about making the key just be the source of getRequiredLimits but I can imagine someone writing some more generic requester that takes a list of of limits in which case the code would be generic and so using the source of the function would not be a valid key.

I also thought about adding a MaxOfSpecificLimitsMixin where you could pass in a list of maxes but I think most code that needs maxes can use MaxLimitsTestMixin

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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.