tests/conftest: Don't subclass dbusmock.DBusTestCase

For historical reasons, dbusmock is written for Python's unittest
framework and most of what it does is done via @classmethod. We called
it on the subclassed object though, so we got some weird behaviors.

In particular, things like `a = foo.someval` would resolve to the
parent class' `someval` but `foo.someval = a` would set that value on
the subclassed object. This lead, indirectly, to our forked dbus-daemon
not getting terminated.

This needs to be be fixed in dbusmock (see [1]) but for now simply work
around this by instantiating the DBusTestCase class directly.

[1] martinpitt/python-dbusmock#184

tests/conftest.py

@@ -11,27 +11,17 @@
 from tests import PortalMock
 
 
-class SessionBusMock(dbusmock.DBusTestCase):
-    def __init__(self):
-        super().__init__()
-        self._dbus_con = None
-
-    @property
-    def dbus_con(self) -> "dbus.Bus":
-        return self._dbus_con
-
-
 @pytest.fixture()
-def session_bus() -> Iterator[SessionBusMock]:
+def session_bus() -> Iterator[dbusmock.DBusTestCase]:
     """
     Fixture to yield a DBusTestCase with a started session bus.
     """
-    bus = SessionBusMock()
+    bus = dbusmock.DBusTestCase()
     bus.setUp()
     bus.start_session_bus()
     con = bus.get_dbus(system_bus=False)
     assert con
-    bus._dbus_con = con
+    setattr(bus, "dbus_con", con)
     yield bus
     bus.tearDown()
     bus.tearDownClass()