Remove START_AFTER_TEST

This is causing the gnome_apps test to be skipped if gnome_install fails, which is not what we want. (Example: https://openqa.gnome.org/tests/970#dependencies)

The idea was to ensure tests run in a specific order, but it doesn't actually matter what order they run in. As long as WORKER_CLASS is set then they will all still run sequentially on the correct runner.