Test texture mix weights separately

[greggman]

Before this change, the texture builtin utils would query how the local GPU mixes texture mip levels. It expected that to be close to a linear interpolation which is true for most GPUs. The known exceptions are Mac AMD and Mac Intel in a compute stage.

It would check how far from linear interpolation the weights are. If the weights were too far off linear interpolation it would assert.

With this change, those asserts for linear interpolation are moved from all the tests that use these weights to a separate test.

This allows the texture builtin tests to run further on devices that have bad mix weights with the new test at least documenting a particular GPU's failure to implement linear interpolation between mip levels.

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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. (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.