Having /etc/securetty in test containers prevents root from logging into
them:
```
Jan 31 10:15:11 systemd-testsuite login[69]: pam_securetty(login:auth): access denied: tty 'pts/0' is not secure !
Jan 31 10:15:11 systemd-testsuite login[69]: FAILED LOGIN 1 FROM pts/0 FOR root, Authentication failure
```