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>