wpt: check for wpt-prefs.json in "./servo" sub-directory
The current working directory when running on WPT runners is not the
folder with the servo binary, but a parent folder into which the servo
nightly tar is extracted. This means the binary is $PWD/servo/servo
and resources directory in in $PWD/servo/resources, so the current
hardcoded relative path resources/wpt-prefs.json will not work on WPT
runners.
This is causing crashes in WPT runner:
https://github.com/web-platform-tests/wpt/runs/29284407168
https://community-tc.services.mozilla.com/tasks/J7MqNwJGQRSkIWlvB5ktPQ/runs/0/logs/live/public/logs/live.log
Signed-off-by: Mukilan Thiyagarajan mukilan@igalia.com