Fix missing assert

Author: DRMacIver

hypothesis-python/tests/cover/test_conjecture_engine.py

@@ -1939,5 +1939,5 @@ def test_function(data):
         assert data.status == Status.VALID
         for n in [2, 1, 0]:
             prefix_data = runner.cached_test_function(hbytes(n))
-            prefix_data is Overrun
+            assert prefix_data is Overrun
         assert call_count[0] == 1