I think for the vast majority of our tests, it doesn't matter.  But it's conceivable it might for one testing that the background of the figure is transparent for example.  It probably saves some time, which is important when considering how long the tests currently take to run.

Note also that for PDF comparisons, Ghostscript never gives an alpha channel, so it's completely redundant there.

Maybe it should be a kwarg `compare_alpha` (defaulting to False)?  I wouldn't want that to hold up this PR because of it.