If run_qemu() exits with non-zero, this either meant that QEMU was not
available (which should be a SKIP) or that QEMU timed out if $QEMU_TIMEOUT was
set (which then should be a FAIL).
Limit the exit code of run_qemu() to QEMU availability only, and track timeouts
separately through the new $TIMED_OUT variable, which is then checked in
check_result_qemu().
Do the same for $NSPAWN_TIMEOUT and run_nspawn() so that nspawn and QEMU work
similarly.
In this test /etc/fstab is replaced by -.mount unit. This causes
systemd-remount-fs.service to not remount / rw, which in turn causes various
failures becuase /var is not writable. In particular
systemd-tmpfiles-setup.service reports many failures. This is something
to possibly fix on its own (see https://github.com/systemd/systemd/issues/791);
in the meanwhile let's fix this test so that it doesn't fail, since the
point of the test is to check aliases on mount units, and not a ro root.
