WGSL textureLoad tests for external texture

[greggman]

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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.