Add chained and compound swizzle assignment tests

[sudonatalie]

The compound swizzle assignment tests are passing with the WIP dawn implementation at: https://dawn-review.googlesource.com/c/dawn/+/290357

Chained swizzle assignment is not yet implemented.

Issue: #4579

---

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 f09941a):

-webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_vars:* - 13 cases, 104 subcases (~8/case)
-webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_chained:* - 1 cases, 1 subcases (~1/case)
-webgpu:shader,execution,statement,swizzle_assignment:swizzle_compound_assignment:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_vars:* - 16 cases, 128 subcases (~8/case)
+webgpu:shader,execution,statement,swizzle_assignment:swizzle_compound_assignment:* - 4 cases, 32 subcases (~8/case)
-TOTAL: 280573 cases, 2321497 subcases
+TOTAL: 280578 cases, 2321551 subcases