tests: don't fail directory CI if lvm2-monitor.service fails

tests: don't fail directory CI if lvm2-monitor.service fails

This service sometimes fails in CI for the arch:directory combination, e.g.

2025-01-21T12:07:33.4370559Z [   14.125731] mkosi-check-and-shutdown.sh[419]: + systemctl --failed --no-legend
2025-01-21T12:07:33.4380022Z [   14.126655] mkosi-check-and-shutdown.sh[420]: + tee /failed-services
2025-01-21T12:07:33.4714973Z [   14.160448] mkosi-check-and-shutdown.sh[420]: ● lvm2-monitor.service not-found failed failed lvm2-monitor.service
2025-01-21T12:07:33.4751355Z [   14.164135] mkosi-check-and-shutdown.sh[418]: + [[ ! -s /failed-services ]]

This is the first time this appears in the partcular boot's logs and we always
ignore it and carry on. Let's sidestep checking whether it is this particular
error by masking the unit.

A possible reason might be that it gets pulled in by udev if a test using LVM
runs first.