Add a scenario test for the new App constructor parameter

tests/scenario_tests/test_app.py

@@ -56,6 +56,24 @@ def test_listener_registration_error(self):
         with pytest.raises(BoltError):
             app.action({"type": "invalid_type", "action_id": "a"})(self.simple_listener)
 
+    def test_listener_executor(self):
+        import concurrent.futures
+
+        class TestExecutor(concurrent.futures.Executor):
+            """A stupid executor that does nothing."""
+
+            pass
+
+        executor = TestExecutor()
+        app = App(
+            signing_secret="valid",
+            client=self.web_client,
+            listener_executor=executor,
+        )
+
+        assert app.listener_runner.listener_executor == executor
+        assert app.listener_runner.lazy_listener_runner.executor == executor
+
     # --------------------------
     # single team auth
     # --------------------------