Support *.worker.js tests

[ErichDonGubler]

worker.js spec. files are a common method of authoring WPT tests. We should support them, both because (1) it's likely that WebGPU CTS upstream may generate these in the future, and (2) moz-webgpu-cts has ambitions for supporting wider use cases in the far future.

We don't add any.js tests to the scope of this particular issue; that's reserved for #123. This issue should come first, since the logic is simpler (easier to author and review), and we're likely to build on top of the changes we need to make here in that issue.