Add chgrp/delete Python tests (assist #12452 and #12544).

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. It also adds the test from https://trac.openmicroscopy.org.uk/ome/ticket/12544.

Outstanding questions:
1) 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.

To test: verify that code makes sense and that the Python integration tests are still green.

/cc @joshmoore, @mtbc
It might be worth adding a comment here and/or expanding the method name given that it can only create a couple of types of links. As it could be expanded to other links maybe a comment giving its current limitations. It might also be worth catching exceptional usage?