Set the AT_SPI_CLIENT variable in the tests

This ensures the test itself is not recognized as a client
by the registry. I'm not sure how that happens since we are
not loading gtk, but it does. Not being a real gtk client with
a mainloop we wasn't answering dbus calls and causing the registry
to hang and fail.

src/sugar3/test/unittest.py

@@ -35,10 +35,14 @@ class UITestCase(unittest.TestCase):
         self._orig_level = logger.getEffectiveLevel()
         logger.setLevel(logging.DEBUG)
 
+        os.environ["AT_SPI_CLIENT"] = "yes"
+
     def tearDown(self):
         logger = logging.getLogger()
         logger.setLevel(self._orig_level)
 
+        del os.environ["AT_SPI_CLIENT"]
+
     @contextmanager
     def run_view(self, name):
         view_path = os.path.join("views", "%s.py" % name)