Add vec function var uniformity cases

[sudonatalie]

Add cases to the uniformity tests for vector function variables, including cases to test the uniformity rules for swizzle assignment as specified in: gpuweb/gpuweb#5268

All tests pass in Tint following this fix:
https://dawn-review.googlesource.com/c/dawn/+/304176

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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 d6bad03):

-webgpu:shader,validation,uniformity,uniformity:function_variables:* - 201 cases, 201 subcases (~1/case)
+webgpu:shader,validation,uniformity,uniformity:function_variables:* - 246 cases, 246 subcases (~1/case)
-TOTAL: 281182 cases, 2322747 subcases
+TOTAL: 281227 cases, 2322792 subcases