test: mask the mdmonitor.service

test: mask the mdmonitor.service

It's pulled in by one of the udev rules (63-md-raid-arrays.rules) and it
fails every time, because there's no valid email address in
/etc/mdadm.conf:

[    5.778153] testsuite-64.sh[403]: mdadm: array /dev/md/mdmirror started.
[    5.819137] kernel: md/raid1:md127: not clean -- starting background reconstruction
[    5.819141] kernel: md/raid1:md127: active with 2 out of 2 mirrors
[    5.819159] kernel: md127: detected capacity change from 0 to 129024
[    5.821950] kernel: md: resync of RAID array md127
...
[    5.887192] mdadm[424]: mdadm: No mail address or alert command - not monitoring.
[    5.890772] systemd[1]: Starting mdmonitor.service...
[    5.891718] systemd[1]: Started mdmonitor.service.
[    5.892570] systemd[1]: mdmonitor.service: Main process exited, code=exited, status=1/FAILURE
[    5.892618] systemd[1]: mdmonitor.service: Failed with result 'exit-code'.

And as we (re)assemble the MD devices multiple times, this gets quite
noisy, especially since we later start hitting the service start rate
limit.

Fedora has the mdmonitor.service patched, so it won't start without
/etc/mdadm.conf being present, but Arch uses the upstream unit which
doesn't have such guard.

Let's just mask the service completely, which replaces all that noise
with one warning:

[    6.553583] testsuite-64.sh[294]: + udevadm wait --settle ...
[    6.580700] systemd[1]: sys-devices-virtual-block-md127.device: Failed to enqueue SYSTEMD_WANTS job, ignoring: Unit mdmonitor.service is masked.