Allow removing VMs based on multiple prefixes at once. Removing them
separately doesn't handle all the dependencies (default_netvm, netvm)
correctly. This is needed for backup compatibility tests, where VMs are
created with `test-` prefix and `disp-tests-`. Additionally backup code
will create `disp-no-netvm`, which also may need to be removed.
Changed the inheritance hierarchy:
1. Renamed `SystemTestsMixin` to `SystemTestCase`
2. `SystemTestCase` is a child of `QubesTestCase`
3. All classes extending the prior `SystemTestsMixin` now just extend `object`