Add more cases to canvas reftest with device loss

[kainino0x]

Test for gpuweb/gpuweb#4606
Expected results need careful review.
See discussion here about what the spec says: #3669 (review)

Issue: #3251

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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]

cc @perryuwang

[perryuwang]

So my understanding is as follows, is it accurate?
When canvas is configured but getCurrentTexture is not called:

• Display a transparent image if configure()d as transparent.
• Display a black image if configure()d as opaque.

[kainino0x]

I think so. It should be the same regardless of whether getCurrentTexture is called: both configure() and getCurrentTexture() wipe the canvas to blank (transparent or opaque depending on the configuration).

[kainino0x]

gpuweb/gpuweb#4606 was resolved, and this test should still be correct.

[toji]

A couple of nits but LGTM.

[toji]

Why not initialize threw to false instead of setting it here?

[toji]

µNit: It's a little strange to have some of these colors be 6 hex digits and others be 8? I feel like the intent might be a bit clearer if we explicitly put the last FF/00 on them to indicate the expected alpha state?