Improve test coverage

[astrofrog]

The current test coverage is around 50%, so we should work on improving that:

https://coveralls.io/r/astropy/pyregion

Of course, we shouldn't try and improve coverage of e.g. kapteyn_celetial.py since this is an external package (and see #43 for a suggestion of removing it in favor of using astropy.coordinates).

[cdeil]

Current test coverage is at 63%:
https://coveralls.io/github/astropy/pyregion

Especially pyregion/mpl_helper.py is at 0%, if someone could add even one test that does MPL plotting, that would be very valuable to have.

Other files we should focus on are pyregion/region_to_filter.py and pyregion/core.py where it's easy to test many lines with just a few high-level test that make sure key functionality is working and keeps working.

I'm assigning this to v1.3.