TEST-15: add test for transient units with drop-ins

TEST-15: add test for transient units with drop-ins

We want to test four things:
- that the transient units are successfully started when drop-ins exist
- that the transient setings override the defaults
- the drop-ins override the transient settings (the same as for a normal unit)
- that things are the same before and after a reload

To make things more fun, we start and stop units in two different ways: via
systemctl and via a direct busctl invocation. This gives us a bit more coverage
of different code paths.