TEST-17-UDEV: rotate journal before checking

TEST-17-UDEV: rotate journal before checking

Otherwise, journal files may be rotated during checking journal entries
and the main system journal file may not be loaded:
```
[  350.372652] TEST-17-UDEV.sh[5841]: + test b253:2 = b253:2
[  350.373288] TEST-17-UDEV.sh[5841]: + [[ 1 == \1 ]]
[  350.373722] TEST-17-UDEV.sh[5841]: + journalctl -n 1 -q -u systemd-udevd.service --invocation=0 --grep 'Found inotify watch .*457'
[  350.374534] TEST-17-UDEV.sh[5970]: Journal file /run/log/journal/edc1fb58daff00ad89d6c8d9689bf172/system.journal is truncated, ignoring file.
[  350.375415] TEST-17-UDEV.sh[415]: + echo 'Subtest /usr/lib/systemd/tests/testdata/units/TEST-17-UDEV.watch.sh failed'
```

Fixes #38559.