test__semaphore.py: add test_release_twice()

greentest/test__semaphore.py

 import greentest
+import gevent
 from gevent.coros import Semaphore
 
 
@@ -10,6 +11,16 @@ class TestTimeoutAcquire(greentest.TestCase):
         result = s.acquire(timeout=0.01)
         assert result is False, repr(result)
 
+    def test_release_twice(self):
+        s = Semaphore()
+        result = []
+        s.rawlink(lambda s: result.append('a'))
+        s.release()
+        s.rawlink(lambda s: result.append('b'))
+        s.release()
+        gevent.sleep(0.001)
+        self.assertEqual(result, ['a', 'b'])
+
 
 if __name__ == '__main__':
     greentest.main()