I added tests for the presence client interface. But the presence

client interface, I realize, is really boring and not worth testing
much.  More interesting, I guess, is the mockdbus module.  This will
need some more extension to be used by a server (I think), like the
presence server.

sugar/presence/test_presence.txt

@@ -0,0 +1,26 @@
+This is a test of presence.
+
+To test this service we will start up a mock dbus library:
+
+    >>> from sugar import mock_dbus
+    >>> import dbus
+    >>> pres_service = mock_dbus.MockService(
+    ...     'org.laptop.Presence', '/org/laptop/Presence', name='pres')
+    >>> pres_service.install()
+    >>> pres_interface = dbus.Interface(pres_service, 'org.laptop.Presence')
+
+Then we import the library (second, to make sure it connects to our
+mocked system, though the lazy instantiation in get_instance() should
+handle it):
+
+    >>> from sugar.presence import PresenceService
+    >>> ps = PresenceService.get_instance()
+    >>> pres_interface.make_response('getServices', [])
+    >>> ps.get_services()
+    Called pres.org.laptop.Presence:getServices()
+    []
+    >>> pres_interface.make_response('getBuddies', [])
+    >>> ps.get_buddies()
+    Called pres.org.laptop.Presence:getBuddies()
+    []
+