TST: Add unit tests for plotting helper functions

[nawtrey]

As mentioned in 4. of this comment, there are several plotting functions that could use unit tests in plotting.py. At the moment the plotting test doesn't properly "test" anything, they primarily flush the code paths to make sure there aren't any errors raised. But adding some unit tests would be a good start.

[nawtrey]

I should note that once this issue is resolved, a new issue should be opened to address more rigorous testing of the public plotting functions.

[nawtrey]

_plot_panel(cbt=True) needs to be tested for more complex graphs like the 8-state model in the kda-examples repo. It currently "works", but we don't have any tests that actually check if the correct nodes are being colored in the figure generated by plotting._plot_panel(). It's never easy to pull data out of matplotlib figures, but it would be nice to retrieve the node indices and their respective colors directly from the subplots and compare them to the expected values.