Minor change in test string

But we do not use a Python counterpart to expectPromise.

Replicates graphql/graphql-js@30d51c3

Author: Cito

tests/execution/test_map_async_iterable.py

@@ -177,10 +177,11 @@ async def __anext__(self):
         assert await anext(doubles) == 4
 
         # Throw error
-        with raises(RuntimeError, match="Ouch") as exc_info:
-            await doubles.athrow(RuntimeError("Ouch"))
+        message = "allows throwing errors when mapping async iterable"
+        with raises(RuntimeError) as exc_info:
+            await doubles.athrow(RuntimeError(message))
 
-        assert str(exc_info.value) == "Ouch"
+        assert str(exc_info.value) == message
 
         with raises(StopAsyncIteration):
             await anext(doubles)