ref tests should match the Gecko naming scheme of `test.html` and `test-ref.html`

[zwarich]

The Gecko ref tests have the naming scheme of test.html and test-ref.html. This is less ambiguous than our current naming scheme of test-a.html and test-b.html, so we should probably switch to it.

[zwarich]

Sorry, I meant test_a.html and test_b.html.

[frewsxcv]

Even though test-ref has been removed, we still have tests that should be renamed in ./tests/wpt/mozilla/tests/