test: skip the test early if we're built without systemd-homed

test: skip the test early if we're built without systemd-homed

We check for homed stuff in the test itself, but this is way too late,
since we already started a unit that Requires=systemd-homed.service
(testsuite-46.service). For now this doesn't matter, but with #27852
the offending transaction is dropped from the job queue, making the test
fail.

Spotted in #27852 in Ubuntu CI.