Test indexing of a matrix using non-const index

[sagudev]

I think there is no such test in CTS and I need it to test gfx-rs/wgpu#6362. Test is based on code from webgpu:shader,execution,expression,access,array,index:runtime_sized:*.

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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. Passes in EDGE, not working on Firefox due to missing support in wgpu (reason for writing this).
• 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 in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

[sagudev]

Is anything blocking this?