builddir and workdir properties should be strings, not tuples

With eb8526ce0, the builddir and workdir properties became tuples, not strings, caused by a trailing comma.

The change to fix this bug should be unproblematic (remove said comma), but the unit test i wrote for this introduces a dependency on mock 0.8.0. If this isn't acceptable (I suspect it isn't), could somebody give me a pointer on alternative approaches?

The mock 0.8.0 specific features I use are assert_has_calls and the call() object.