test__pywsgi.py: cleanup before test case end

--HG--
extra : transplant_source : %B45%80%3F%83%91%E3%10-%D9%D0%9B%2A%9F%98%D5%82lc%B0

greentest/test__pywsgi.py

@@ -882,6 +882,7 @@ class TestLeakInput(TestCase):
         fd.write("GET /leak-frame HTTP/1.0\r\nConnection: close\r\n\r\n")
         d = fd.read()
         assert d.startswith("HTTP/1.0 200 OK"), "bad response"
+        self._leak_environ.pop('_leak')
 
 
 del CommonTests