Auto merge of #12547 - jryans:wpt-firefox, r=jgraham

Allow running WPT in Firefox

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When working with WPT tests, I find it convenient to compare the results of different browsers / engines. By installing a few more Python packages, we can easily test against Firefox from a Servo checkout.

As noted in the updated README.md, this change allows you to check WPT tests in Firefox by adding `--product firefox` to the `./mach test-wpt` command.

r? @jgraham

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] These changes do not require tests because only the set of test harness packages is changed

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