Mach update-manifest command can fail with mozilla metadata

[gterzian]

See #22769 (review)

It looks like a previous call to ./mach update-manifest had added a /_mozilla/mozilla/ entry to tests/wpt/mozilla/meta/MANIFEST.json , which then made test-tidy fail in the following way:

File "/servo/tests/wpt/web-platform-tests/tools/manifest/item.py", line 68, in __init__
    assert url[0] != "/"

Subsequent calls to update-manifest failed silently.

The fix was to explicitly run ./mach test-wpt --manifest-update tests/wpt/mozilla/tests/mozilla/ --manifest tests/wpt/mozilla/meta/MANIFEST.json.

We could update update-manifest to have it deal with such a situation automatically.

[jdm]

I suspect this is a specialized issue where the PR was made before a WPT sync that updated the manifest format, and then the manifest update code did not see any change in the file in question and therefore did not rebuild its manifest entry. Probably touching the file would have made it work.