ipatests: properly handle journalctl return code
The test test_installation.py::TestInstallMaster::test_selinux_avcs
is failing when no AVCs are detected because it is calling
journalctl --full --grep=AVC--since=yesterday
and the command exits with return code 1.
Call the command with raiseonerr=False to support this case.
Fixes: https://pagure.io/freeipa/issue/8541
Reviewed-By: Mohammad Rizwan Yusuf <myusuf@redhat.com>
Reviewed-By: Francois Cami <fcami@redhat.com>