BUG: avoid spurious python warning when a volume has no display node

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22926 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: #2921 add summary dialog after running test

This was already being generated when running all tests, but
now do it even for a single test button click.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22927 3bd1e089-480b-0410-8dfb-8563597acbee