Move setting AT_SPI_CLIENT to discover

It needs to be called as early as possible before unit tests
has a chance to import gtk.

Hopefully will fix random test failures we are seeing in buildbot.

src/sugar3/test/discover.py

@@ -37,6 +37,7 @@ def main():
 
     os.chdir(args.tests_dir)
     os.environ["TMPDIR"] = temp_dir
+    os.environ["AT_SPI_CLIENT"] = "yes"
 
     try:
         test = unittest.defaultTestLoader.discover(".")

src/sugar3/test/unittest.py

@@ -35,14 +35,10 @@ 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)