documented as being empty and subclasses shouldn't need to call it. This would
make the change backwards incompatible and needs thinking about.
+In Python 2.7 the ``TestSuite`` held references to each ``TestCase``
+after ``TestSuite.run``. In unittest2, ``TestSuite.run`` sets those
+references to None so that the garbage collector can free their memory.
+Subclasses can restore the earlier behavior by overriding