tests: stop using `nobody` in test-udev.pl (#8239)

`nobody` is a special user, whose credentials should be extracted with
`get_user_creds`. `getpwnam` called in `test-udev.pl` is a bit different,
which causes the test to fail with the following error:
```
device '/devices/pci0000:00/0000:00:1f.2/host0/target0:0:0/0:0:0:0/block/sda' expecting node/link 'node'
  expected permissions are: nobody::0600
  created permissions are : 65534:0:0600
permissions: error
add:         ok
remove:      ok
```
The ideal fix would probably be to implement `get_user_creds` in Perl, but in this
PR the issue is simply got around by using `daemon` instead of `nobody`.

Closes https://github.com/systemd/systemd/issues/8196.