Swizzle assignment tests

[sudonatalie]

Add some initial valid swizzle assignment tests on basic var vectors.

These tests pass on my current WIP implementation of the feature locally.

Issue: #4579

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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 8ef7fcf):

+webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment:* - 11 cases, 11 subcases (~1/case)
-webgpu:shader,validation,parse,requires:wgsl_matches_api:* - 8 cases, 8 subcases (~1/case)
+webgpu:shader,validation,parse,requires:wgsl_matches_api:* - 9 cases, 9 subcases (~1/case)
-TOTAL: 285934 cases, 2344827 subcases
+TOTAL: 285946 cases, 2344839 subcases

[alan-baker]

Can you add TODOs or unimplemented test suites for the following:

• Other address spaces (only function is tested currently)
• Chained swizzles
• Pointer to swizzle

[alan-baker]

Is count necessary? Couldn't this just be i?

[alan-baker]

Can you add a boolean vector case or two? You'd need to modify the shader to use a different output type.