ENH : added test on number of large differences between images

Added a test in compare_images to count the number of pixel which
are different by some threshold.  If the number of pixels which
have a difference which exceeds  this threshold is greater than
some maximum, the test fails.

This is to address the observation (see #2707)  that some of the image tests
lost half of their axes labels, but were still passing the RMS
test.