Correct swizzle assignment on pointer tests

[sudonatalie]

The swizzle assignment spec was updated so that creating a SwizzleView from pointer requires support for pointer_composite_access, so CTS tests here are updated to correctly handle the interaction between swizzle_assignment and pointer_composite_access. Execution tests for swizzle assignment via pointer are now skipped if the pointer_composite_access language feature is not available, and validation tests are added to validate both cases.

Bug: crbug.com/534329233

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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 ad05a05):

+webgpu:shader,validation,statement,swizzle_assignment:pointer_swizzle_assignment:* - 8 cases, 8 subcases (~1/case)
-TOTAL: 285236 cases, 2346080 subcases
+TOTAL: 285244 cases, 2346088 subcases