The issue is on Chrome and Firefox on Intel Mac,
when sampling between 2 mip levels using textureSampleLevel in a compute shader, the weights used for mixing are very unexpected. The same issue doesn't happen in Safari TP so there is probably a fix.

For now though, the same issue doesn't happen when using a fragment shader. So, switched to using a fragment shader to look up these weights. This is more appropriate for the current tests because the tests are running in fragment shaders.

Will add an issue to test all stages.

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 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.