New test for 'Security Violation' Bug in PR 1139

In the web UI, we get a 'Security Violation' trying to move
an image to a new Dataset (AFTER doing chgrp).
Turns out this is due to the group Id of the fileset not
being changed by the chgrp.
Dataset names in chgrp.py reflect new test names
Revise Python delete tests to reflect fileset immutability.