tests: Ensure Python assertEventually() function uses correct main context

It should be doing already (or the tests would never work), but make it
clearer in the code that the same `GMainContext` is being iterated as is
being woken up in the rest of the test.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #2887

gio/tests/memory-monitor-dbus.py.in

@@ -73,8 +73,7 @@ try:
             printed on failure.
             '''
             while timeout >= 0:
-                context = GLib.MainContext.default()
-                while context.iteration(False):
+                while self.main_context.iteration(False):
                     pass
                 if condition():
                     break