Add mixed swizzle and indexing tests

[sudonatalie]

Check that a valid mix of swizzles and index works and that an invalid OOB case fails.

---

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 93b8515):

-webgpu:shader,validation,expression,access,vector:concrete:* - 60 cases, 6480 subcases (~108/case)
-webgpu:shader,validation,expression,access,vector:abstract:* - 24 cases, 2592 subcases (~108/case)
+webgpu:shader,validation,expression,access,vector:concrete:* - 60 cases, 6600 subcases (~110/case)
+webgpu:shader,validation,expression,access,vector:abstract:* - 24 cases, 2640 subcases (~110/case)
-TOTAL: 285746 cases, 2344470 subcases
+TOTAL: 285746 cases, 2344638 subcases