Add chgrp/delete Python tests (assist #12452).

This is a PR that moves test outlined in https://trac.openmicroscopy.org.uk/ome/ticket/12452 from @dpwrussell's https://gist.github.com/dpwrussell/6aa271b5c88a106c6f4a to the OmeroPy test suite. This isn't ready for merging, as:
1) I'm not sure if the addition of `mkP()`, `mkD()` etc. to `library.py` is useful? If it is, I'd imagine we could simplify the code of other tests in the suite by re-using those methods.
2) Tests that fail have been marked with `@xfail`. What's tricky, is that I'm not sure if those failures are expected. That should be the main point of this PR.
3) If this becomes a proper PR, then the people also working with Python tests (@sbesson, @ximenesuk) would have to have a look at the overall structure.

/cc @joshmoore, @mtbc

--exclude
If these helpers are useful then I'd also suggest moving the imports out to the module level and being consistent in using them. across these methods. ie `dataset = DatasetI()` rather than `dataset = omero.model.DatasetI()`