Load integration tests from outside of core-admin repository, through
entry points.
Create wrapper for VM object to keep very basic compatibility with tests
written for core2. This means if test use only basic functionality
(vm.start(), vm.run()), the same test will work for both core2 and
core3. This is especially important for app-* repositories, where the
same version serves multiple Qubes branches.
This also hides asyncio usage from tests writer.
See QubesOS/qubes-issues#1800 for details on original feature.
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`
Now failure to load external tests shows in which entry point the error
happened and a useful traceback. The traceback extends from the "try"
statement down to the actual error line, but it does not include the
frames above, ie. from the invocation to the load_tests routine. This is
a limitation of Python itself and usually not a problem.
