python-packages/poezio: Fix tests

Poezio doesn't install the files required for running the tests, but
also while building with Nix we don't end up getting valid shared
objects within the temporary build directory.

So we now running "make test" (which does "py.test -v test/") with a
PYTHONPATH that adds the /poezio directory of the path in site-packages
to make sure that the test runner is able to import the shared objects.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @lancelotsix