test: Don't allocate scope if already running in unit with delegated cgroup

test: Don't allocate scope if already running in unit with delegated cgroup

If we're already running in a unit with delegation turned on, let's
skip allocation of a scope unit and cgroup subroot. This allows journald
to correctly attribute the logs of all subprocesses spawned by tests such
as test-execute to the test-execute service when the test is running in a service.