Add tests_require to setup.cfg and setup.py

[astrofrog]

This could be done similarly to #208, but we should make sure it actually works (in a few simple experiments, I found that while the eggs do get created, the dependencies are not seen by the test process).

[pllim]

Done in #438

[astrofrog]

Just for the record, we actually added a test section in extras_require rather than use tests_require - the latter aren't really well supported by testing tools