test_file_tar: skip if codecs are not available.

The .tar test requires both jpeg & zlib support. If one of the two is
unavailable, run the test for the second one. If both are unavailable,
skip the test.
test_imagefile: handle missing PNG & JPEG encoders gracefully.
Support (most of) test_image_split without PNG.

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.
test_image_draft: skip if no JPEG support.
test_font_pcf: skip if zlib is not available.
test_file_tiff: skip if TIFF is not available.
Split tiff tests so that test_file_tiff tests the builtins, and test_file_libtiff tests only things that depend on libtiff
skipp these test is libtiff support is not included
rename to include libtiff
renamed
Merge pull request #201 from wiredfool/pr199

Skip tests for features that aren't compiled in, includes #199