Do not force inheritance of ExtraTestMixin and QubesTestCase. Instead
provide 'qubes.tests.extra.ExtraTestCase' for external tests. This makes
the API less "magic", easier to understand and apply static analysis
tools on it.
QubesOS/qubes-issues#1800
This way, tests will appear as from 'extra' module.
Besides the move, some minor changes:
- add missing self.qc.unlock_db() in create_vms()
- suffix per-template tests with template name
QubesOS/qubes-issues#1800
