Commit 70b0d2a2 broke unittests on Python 2.4 and 2.5. Turns out that
Python 2.6 and above allow classes to be passed as custom test runners,
whereas earlier versions don't.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>