test: verify our own units (where applicable)

test: verify our own units (where applicable)

This is inspired by one of our internal tests that does pretty much the
same thing. However, it is slightly more convoluted than I'd like it to
be, since I really don't want to duplicate the list of our units in
another place, so we need to, somehow, pass the list from the meson file
to the test script. I originally envisioned this to be a part of the
unit test suite, but this doesn't work for unit files with absolute
paths to binaries, as we'd have to install the build first (maybe using
a chroot would work?).

It doesn't check man pages (since they might not be installed on the
test machine) and also skip recursive dependencies (as that would trip
over issues in files that are not under our direct control), but it
should still cover typos and such.

There are currently two units for which the check had to be disabled -
syslog.socket, as the corresponding syslog.service might not be
installed, and rc-local.service as that's a compat API and the necessary
/etc/rc.d/rc.local file may not (and most likely won't be) present.