test: add test for DynamicUser= + StateDirectory=

test: add test for DynamicUser= + StateDirectory=

Also, tests for DynamicUser= should really run for system mode, as we
allocate from a system resource.

(This also increases the test timeout to 2min. If one of our tests
really hangs then waiting for 2min longer doesn't hurt either. The old
2s is really short, given that we run in potentially slow VM
environments for this test. This becomes noticable when the slow "find"
command this adds is triggered)