ipatests: fix backup and restore
    
    The tests for backup_and_restore check that the ipa-backup command
    compresses the tar file AFTER restarting IPA services by reading the
    output and looking for a pattern with "gzip" before "Starting IPA service."
    
    As the tar file name is randomly created, it sometimes happen that the
    name contains gzip and in this case the test wrongly assumes that
    the gzip cmd was called.
    
    The fix makes a stricter comparison, looking for /bin/gzip.
    
    Fixes: https://pagure.io/freeipa/issue/8170
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>