Install problematic dependencies in before_install

Use the virtualenv --system-site-packages option to inherit these
hard-to-build dependencies into the virtual python environment.

Rationale:
PyGObject and dbus-python turn out to be pretty hard to install using
the virtualenv environment, especially on python3 where the configure
script fails to find the python headers.
Use python-dbusmock for testing

This is better than relying on an external service that may not be
available. Still, the dbusmock library seems somewhat clumsy. It might
be best to write a (very simple) dbus framework.