Add dependency on python-mock

Python Mock (http://www.voidspace.org.uk/python/mock/

) is a mocking
library for Python. It is included in the standard Python distribution
as of Python 3.3, but has been around for quite a while. Debian unstable
contains a version we require.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>