test: install modinfo to test image

test: install modinfo to test image

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
```