gajim: fix tests

`test` directory actually also contains various libraries and broken
tests. Test failure happens when `test/lib/gajim_mocks.py` is attempted
to run as a test.

Upstream only runs no_gui tests:
https://dev.gajim.org/gajim/ci/-/blob/e0f58cfc780c808b54b0170bc494c6e10a68c0a5/.gajim-ci.yml#L18

We now explicitly run only tests from `no_gui` and `unit`.

pkgs/applications/networking/instant-messengers/gajim/default.nix

@@ -59,7 +59,8 @@ python3.pkgs.buildPythonApplication rec {
   checkPhase = ''
     xvfb-run dbus-run-session \
       --config-file=${dbus.daemon}/share/dbus-1/session.conf \
-      ${python3.interpreter} setup.py test
+      ${python3.interpreter} -m unittest discover -s test/unit -v
+    ${python3.interpreter} -m unittest discover -s test/no_gui -v
   '';
 
   # necessary for wrapGAppsHook