Auto merge of #7789 - 6112:master, r=mbrubeck
Auto merge of #7789 - 6112:master, r=mbrubeck

allow `./mach test` to run tests in tests/wpt/mozilla/

Allows running WPT tests in the tests/wpt/mozilla/ directory by using commands such as: 

```
./mach test tests/wpt/mozilla/tests/mozilla/union.html
```

Fixes #7772.

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