Follow-up for
6c2d47d6d3ad25ffd7527c7f4de31457ee1b25d8.
Fixes the following unexpected skip:
```
[ 6.163670] TEST-64-UDEV-STORAGE.sh[596]: + modinfo btrfs
[ 6.164102] TEST-64-UDEV-STORAGE.sh[726]: /usr/lib/systemd/tests/testdata/units/TEST-64-UDEV-STORAGE.sh: line 726: modinfo: command not found
[ 6.164683] TEST-64-UDEV-STORAGE.sh[727]: + echo 'This test requires the btrfs kernel module but it is not installed, skipping the test'
[ 6.165069] TEST-64-UDEV-STORAGE.sh[728]: + tee --append /skipped
[ 6.166801] TEST-64-UDEV-STORAGE.sh[728]: This test requires the btrfs kernel module but it is not installed, skipping the test
[ 6.167177] TEST-64-UDEV-STORAGE.sh[596]: + exit 77
```
mkfifo
mknod
mktemp
+ modinfo
modprobe
mount
mountpoint