Provide valid data for astc textures

[greggman]

This is a temporary fix but it seems valid as something is still tested. The temp fix is to make the simplest block which is a void-extent block (a solid color). Blocks still have a random color so different blocks test filtering between mip levels and between blocks.

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:

• [x] Tests are properly located in the test tree.
• [x] Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• [x] Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• [x] Helpers and types promote readability and maintainability.

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