TST: Add image test for display in fluxcal.py

[pllim]

Currently, this block is not tested:

specreduce/specreduce/fluxcal.py

Line 262 in aa4ceab

if display is True:

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).