/usr/lib/systemd/tests may contain more than the unit tests. For example on
SUSE we also install the integration tests there.
Putting the unit tests in a dedicated directory named 'unit-tests' makes the
layout cleaner.
Note that `run-unit-tests.py` has not been moved so we don't need to adjust
(Fedora) packaging and users also don't need to descend into the subdirectory.
When tests are executed serially (the default), it seems better to launch
the fairly generic test that runs the unittests early in the sequence.
Right now the tests are ordered based on when they were written, but
this doesn't make much sense.