Python Shirt tests don't accurately test command line args

[curiouskiwi]

For each of the command line checks, we are looking for a non-zero exit code, but that will happen if the program crashes with an IndexError when it tries to access argv[1] or argv[2] for example, or with a PIL.UnidentifiedImageError if it tries to open a non-suitable image file.

Our checks currently can't distinguish between a purposeful sys.exit("invalid usage") and one of those errors. Hence green smiles even if the code does not check for such things.

@CarterZenke related to: #196

[Makaze]

Are you aiming to find out if they deliberately called sys.exit() in all these cases? I thought I could solve this by making a second dependency test for zero exit given correct arguments, but I'm not sure it it does what you're suggesting.