Add test of single element swizzle assignment

[sudonatalie]

Single element swizzle assignment currently fails in Tint when the object of the swizzle is also a swizzle (i.e. v.xy.x = 1.0). This appears to be a valid assignment which should be covered by CTS so added it here. Since there didn't seem to be an existing validation test for simple assignment statements, I created a new file with a couple other simple tests. As multi-element swizzle assignment is not yet part of the spec, also added a test to check that it fails.

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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 113679a):

+webgpu:shader,validation,decl,assignment_statement:scalar_assignment:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,validation,decl,assignment_statement:vector_full_assignment:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,validation,decl,assignment_statement:vector_indexed_assignment:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,validation,decl,assignment_statement:vector_swizzle_assignment:* - 3 cases, 3 subcases (~1/case)
-TOTAL: 285738 cases, 2344462 subcases
+TOTAL: 285744 cases, 2344468 subcases

[sudonatalie]

Changed as discussed so v.xy.x = 1.0 fails and switched to a bit of parametrization to neaten that up now that there's multiple failure cases.