[MAINT] change tests to figure testing

[slumnitz]

Most of splot's tests are based on testing whether or not a figure is generated.
This should slowly transition to testing the content of the figure itself as suggested by @martinfleis in #140

One option is to get the attributes from the plotted collection as we do in geopandas https://github.com/geopandas/geopandas/blob/64598b6f0d872baf09657c27a9aa4c87d86d9131/geopandas/tests/test_plotting.py#L169

The other is to keep a reference image in tests and check against that as matplotlib does in some cases https://github.com/matplotlib/matplotlib/blob/ec126b0175c657f0adb5ca635eeabad7c142bb0b/lib/matplotlib/tests/test_colorbar.py#L188-L190

[jGaboardi]

xref pysal/libpysal#533 (comment)

re: plotting tests

I've had success with matplotlib's decorator – image_comparison

We should probably open a specific issue for this.