make test__memleak.py not report a SystemExit error when run with py.test

greentest/test__memleak.py

 import sys
 
-if not hasattr(sys, 'gettotalrefcount'):
-    sys.exit(0)
 
 from greentest import TestCase, main
 import gevent
@@ -49,5 +47,8 @@ class TestQueue(TestCase):
         assert b == c, 'total refcount mismatch: %s' % result
 
 
+if not hasattr(sys, 'gettotalrefcount'):
+    del TestQueue
+
 if __name__ == '__main__':
     main()