make singularity-runtime check by dependency (SOFTWARE-3532)

singularity 3.0.2 gets rid of the singularity-runtime package, but
provides it in the singularity base package

Author: edquist

osgtest/tests/test_380_cvmfs.py

@@ -96,7 +96,7 @@ def test_03_oasis_config(self):
             self.debug_cvmfs(oasis_repo)
 
     def test_04_singularity(self):
-        core.skip_ok_unless_installed('singularity-runtime')
+        core.skip_ok_unless_installed('singularity-runtime', by_dependency=True)
         core.skip_ok_unless_installed('cvmfs')
         core.skip_ok_unless_installed('cvmfs-keys', by_dependency=True)
         singularity_repo = 'singularity.opensciencegrid.org'