Add CTS for texture_and_sampler_let language feature

[alan-baker]

CTS for https://github.com/gpuweb/gpuweb/pull/5389

Issue: #

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Requirements for PR author:

• [x] All missing test coverage is tracked with "TODO" or .unimplemented().
• [x] New helpers are /** documented */ and new helper files are found in helper_index.txt.
• [x] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• [x] 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.

[alan-baker]

These tests LGTM but maybe we should also add some coverage via let in an execution test somewhere (like your textureDimensions change)? Otherwise for Tint we won't hit the part of the compiler that actually needs to do something to handle these things.

I've modified the execution tests for textureDimensions to use a let if supported.