test-runner: Disconnect display-opened signal on destruction

When running multiple tests at once (with --all) as in the installed-tests cases, we may open and close the display multiple times, this leads to setting the alarm filter each time that the x11 display is opened (causing a critical error) because we never disconnect from the ::x11-display-opened signal.

So disconnect from the signal on test destruction, to avoid this to be emitted multiple times.

And also fix the installed test files so that they will work with gnome-desktop-testing-runner