Fixed some pathnames in `drugs` test.

I've fixed some pathnames in the `drugs` test.

Not sure if you noticed, but the first drug in the dataset doesn't even pass the test.  The number of *atoms* in the ffxml file you generate for ZINC03873936 is even different from the number of atoms in the GAFF mol2 file.  This molecule has 68 atoms, but only 57 atoms are processed by the gaff2xml conversion.