Add test for PixelRegion MPL plot

This plot code is untested:
https://coveralls.io/builds/18587542/source?filename=regions%2Fcore%2Fcore.py#L311

@sushobhana - It looks like you added two months ago.

Can you please add a test that executes it once?

I know that testing plotting code is hard, but a test that executes this code once is helpful even if it doesn't have any asserts, because frequently we find that code in the regions package or MPL or something else changes, and things break without us noticing. So just executing makes sure that no errors or warnings are raised. You could leave a `#TODO: add asserts on output` on the last line if you want to be nice to future maintainers.

You're probably aware, but you can get the coverage locally for any tests / files you like, in this case I would use
```
pip install pytest-cov
pytest -v --cov-report=html --cov=regions.core.core regions/core
```

Start by reading the PixelRegion plotting code in regions/core/core.py at the linked coverage location, then inspect the existing tests under regions/core. Add a test that executes the MPL plot once without errors or warnings, and run pytest -v --cov-report=html --cov=regions.core.core regions/core to confirm the code is covered.

Written by the indexing model from the issue text.