While it is useful, it is questionable when and how to implement. If C++ is used, perhaps Boost Test Library is a good choice to validate various functions in pihmLIBS and alike. However if it is possible to move as much as possible duplicate functionality out of C++ QGIS plugin then, perhaps, unit testing can be implemented in Python or whatever is available for sextante.
While it is useful, it is questionable when and how to implement. If C++ is used, perhaps Boost Test Library is a good choice to validate various functions in pihmLIBS and alike. However if it is possible to move as much as possible duplicate functionality out of C++ QGIS plugin then, perhaps, unit testing can be implemented in Python or whatever is available for sextante.