If PNG (zlib) support is disabled, use .pcx for test_image_split which
is unconditional. Since it does not support RGBA, skip the RGBA test.
Otherwise, just use PNG and do the full test.
This is Christoph Gohlke's test suite from his personal PIL package found
at http://www.lfd.uci.edu/~gohlke/pythonlibs/.
This is just to bring it in as a separate commit. Future commits will align
it with Pillow.