Chgrp python tests

This PR is on top of @ximenesuk PR https://github.com/openmicroscopy/openmicroscopy/pull/1123, now that we have tests running with server changes.

Added various assertions after the chgrp step of each test.
Removed Datasets from some tests that were not testing Datasets.
Move doc strings into test method body.

Several tests are failing (see commits).

The last 2 tests in chgrp.py that Colin added appear to be duplicates of previous tests above, just named differently. Maybe these have been copied & pasted and are due to be modified? If not, then they could be removed.