TST: Add image test for display in fluxcal.py

[pllim]

Currently, this block is not tested:

https://github.com/astropy/specreduce/blob/aa4ceabbfb69595ac58c2ff29a220aabe1e37f68/specreduce/fluxcal.py#L262

Since it is plotting, you might want to write a test where you compare the image, like how astropy.visualization does it. That would require additional CircleCI setup and a place to host the truth image(s).