WGSL validation tests for textureSampleBias

[greggman]

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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.

[greggman]

I wasn't sure what to do about the abstract-int thing. I passed in 15.99 because it's mentioned in the spec but BigInt(15.99) throws with "not in integer". I could do Math.round or Math.ceil or something in Type.create but decided on acceptsFloat as one solution.

[ben-clayton]

Sorry for the delay in reviewing. I can't navigate github's UI.