Loosen Viewport validation tests to match spec update

[toji]

Should not be landed until gpuweb/gpuweb#5025 has been merged into the spec.

Updates the viewport validation tests to match the new behavior that allows viewports to extend beyond the render attachment bounds.

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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]

I'll mark it as draft to prevent it from being merged accidentally.

[kainino0x]

For each of these we should ideally test two cases that are just an epsilon apart, one valid and one invalid. (see nextAfterF32 in math.ts)

and in order to make it clear that those test cases are actually doing what we expect, it would help to put _success as a parameter, instead of computing it in the test