"Unit" tests for PDB functions accessible by Script-Fu

This is a simple Unit-test-look-alike approach to test the functions available in Script-Fu. Currently, only the gimp-brush-* stuff is done (or at least each function is called at least once).

Everything is based on bunny-test, a framework available from http://community.schemewiki.org/?bunny-test and simple enough to be used with TinyScheme (more complete stuff relies on syntax not available in this implementation).

One bug has been identified this way already, gimp-brush-rename took an empty string for the new name.