gen_wpt_cts_html: Add support for filtering the list of variants

[kainino0x]

Adds a new sample config file gen_wpt_cfg_withsomeworkers.json.

Note building this is really slow (it iterates the whole test tree four times) but it's still useful to have prebuilt and verified on presubmit, so I'm just including it in the build. Hopefully it's not prohibitively slow for anyone's local development (they should not need to run the whole npm test/npx grunt pre build often, hopefully). On my nice fast mac, npm test CPU time goes from 267s to 307s, but wall time goes from 69s to 72s thanks to parallelization.

To run just this one step: ./tools/gen_wpt_cts_html tools/gen_wpt_cfg_withsomeworkers.json
Running locally, the result files seem to be as expected (out-wpt/cts-withsomeworkers.https.html and gen/webgpu_variant_list_withsomeworkers.json).

The list of tests here is actually a superset of those [used by chromium](https://source.chromium.org/chromium/chromium/src/+/main:third_party/dawn/webgpu-cts/worker_test_globs.txt]; I don't remember why we don't run all of these.

Issue: #4098
cc @ErichDonGubler

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Requirements for PR author:

• [n/a] All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• [n/a] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• [n/a] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• [n/a] Tests are properly located in the test tree.
• [n/a] 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.