Explain default Python tests.

The default top-level Python test builds exclude broken and long running tests. This update to the developer docs explains that.

See https://github.com/openmicroscopy/openmicroscopy/pull/3241

--no-rebase
Independent of this PR, but this line is actually incorrect. `pytest` should only be required if running `py.test` directly else `setup.py` will install it as a prerequisite.