[TEST] ensure that wsinfer functions without openslide or without tiffslide

[kaczmarj]

a test should be added that tests whether wsinfer still works if openslide is not installed or tiffslide is not installed. one of these must be installed, so the test should also fail if neither is detected.

[kaczmarj]

i tried doing this with mock from unittest, but i can't... we could add a test in bash that uninstalls tiffslide and makes sure the command line still works, then does the same with openslide...