Close env after test (#334)

Author: naeioi

tests/helpers.py

@@ -42,3 +42,6 @@ def step_env_with_gym_quirks(test_case,
         # Roundtrip serialization
         round_trip = pickle.loads(pickle.dumps(env))
         assert round_trip.env.spec == env.env.spec
+        
+   if not spec.id in KNOWN_GYM_CLOSE_BROKEN:
+        env.close()