test "signal process outside container": skip instead of fail if there is no getty

 ✗ signal process outside container
   (in test file run/ch-run_uidgid.bats, line 138)
     `ch-run $UID_ARGS $GID_ARGS $CHTEST_IMG -- /test/signal_out.py' failed
   no ID arguments

   Traceback (most recent call last):
     File "/test/signal_out.py", line 27, in <module>
       pdata = subprocess.check_output(["pgrep", "-nl", "getty"])
     File "/usr/lib/python3.6/subprocess.py", line 336, in check_output
       **kwargs).stdout
     File "/usr/lib/python3.6/subprocess.py", line 418, in run
       output=stdout, stderr=stderr)
   subprocess.CalledProcessError: Command '['pgrep', '-nl', 'getty']' returned non-zero exit status 1.