Clean step now removes entire build directory.

This is the only way to ensure that we cleaned the previous test
run's build completely. On certain generators
the ADDITIONAL_MAKE_CLEAN_FILES property has no effect.

The working directory for the driver script was also changed to be
The source directory of the test subproject.
Merge pull request #35 from smspillaz/cmake-unit.item_33

Clean step now removes entire build directory.