Add more swizzle assignment tests

[sudonatalie]

Add tests for swizzle assignment on vectors with address space private, workgroup and storage, including via a pointer.

These currently fail in dawn, but do pass with my local WIP implementation of swizzle assignment.

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 b92fecd):

-webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_other_vars:* - 1 cases, 1 subcases (~1/case)
-webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_chained:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_other_vars:* - 2 cases, 2 subcases (~1/case)
+webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_chained:* - 1 cases, 1 subcases (~1/case)
-TOTAL: 285981 cases, 2344958 subcases
+TOTAL: 285982 cases, 2344959 subcases

[alan-baker]

Would it make sense to combine these and the pointer tests into swizzle_assignment_local_var (and rename to vars).

I notice storage is only covered in the pointer version below. Just feels like there's very little coverage with this split.