Refactor copyExternalImageToTexture tested formats.

[greggman]

Rather than have a list of allowed formats, the new spec allows all regular texture formats that are renderable and not compressed, not snorm, not integer.

gpuweb/gpuweb#5289
gpuweb/gpuweb#5299
gpuweb/gpuweb#5323

Note: with the change in the spec to be "color renderable+unorm/float", that means feature: "rg11b10ufloat-renderable" enabling copyExternalImageToTexture for rg11b10ufloat

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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.

[kainino0x]

Technically blocked on gpuweb/gpuweb#5333 but LGTM