Change default TIFF output compression to "zip", it is better than lzw.
Make testtex able to force interp and mip modes.
Add testsuite tests to test each one individually (other than the defaults,
which are covered by existing tests).

This makes it easier to debug the texture system after changes -- if just
one of these tests breaks, it's easy to know we've introduced a bug in a
specific part of the code that deals with that case.