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 5bc7e84):

+webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_local_var:* - 13 cases, 13 subcases (~1/case)
+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_pointer:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,execution,statement,swizzle_assignment:swizzle_compound_assignment:* - 1 cases, 1 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: 285935 cases, 2344828 subcases
+TOTAL: 285953 cases, 2344846 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

[sudonatalie]

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

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

Done and updated tracking bug

[sudonatalie]

Fixed TODOs to make the linter happy and a placeholder validation test file

[sudonatalie]

Fixed TODOs to make the linter happy and a placeholder validation test file

Generators didn't seem to like an empty test file so removed that, still tracking need for validation tests in Issue #4579