image comparison tests have to be run with matplotlib builtin rcdefaults

 - see e.g. what the mpl people do:
   https://github.com/matplotlib/matplotlib/blob/b7f7ddd94d6520908e0463d80103e49eb6c91be4/lib/matplotlib/tests/__init__.py#L24
 - the defaults have to be set regardless how the test is started (e.g.
   directly executing a test file), so the only solution i saw was to
   wrap it around the used compare_images() function itself
 - see #603
 - also some refactoring and removing of duplicated code