CTS for linear_indexing language feature

[alan-baker]

• Execution tests for new builtins
• Validation tests for new builtins

No tests yet for the API validation.

Issue: #

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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 58d4b11):

-webgpu:shader,validation,parse,requires:wgsl_matches_api:* - 9 cases, 9 subcases (~1/case)
+webgpu:shader,validation,parse,requires:wgsl_matches_api:* - 10 cases, 10 subcases (~1/case)
-webgpu:shader,validation,shader_io,builtins:stage_inout:* - 464 cases, 464 subcases (~1/case)
-webgpu:shader,validation,shader_io,builtins:type:* - 58 cases, 2320 subcases (~40/case)
+webgpu:shader,validation,shader_io,builtins:stage_inout:* - 496 cases, 496 subcases (~1/case)
+webgpu:shader,validation,shader_io,builtins:type:* - 62 cases, 2480 subcases (~40/case)
-webgpu:shader,validation,shader_io,builtins:reuse_builtin_name:* - 145 cases, 145 subcases (~1/case)
+webgpu:shader,validation,shader_io,builtins:reuse_builtin_name:* - 155 cases, 155 subcases (~1/case)
-webgpu:shader,validation,shader_io,invariant:valid_only_with_vertex_position_builtin:* - 58 cases, 58 subcases (~1/case)
+webgpu:shader,validation,shader_io,invariant:valid_only_with_vertex_position_builtin:* - 62 cases, 62 subcases (~1/case)
-TOTAL: 286017 cases, 2344994 subcases
+TOTAL: 286068 cases, 2345201 subcases