tests: Add capture_event() helper.

For many actions, we make a single call to
send_event, and it's kind of heavy now to
properly assert we made one call, and we
don't need to exercise all the tornado code
to prove that the action was written correctly.

zerver/lib/test_helpers.py

@@ -109,6 +109,28 @@ def tornado_redirected_to_list(lst: List[Mapping[str, Any]]) -> Iterator[None]:
     yield
     event_queue.process_notification = real_event_queue_process_notification
 
+class EventInfo:
+    def populate(self, call_args_list: List[Any]) -> None:
+        args = call_args_list[0][0]
+        self.realm_id = args[0]
+        self.payload = args[1]
+        self.user_ids = args[2]
+
+@contextmanager
+def capture_event(event_info: EventInfo) -> Iterator[None]:
+    # Use this for simple endpoints that throw a single event
+    # in zerver.lib.actions.
+    with mock.patch('zerver.lib.actions.send_event') as m:
+        yield
+
+    if len(m.call_args_list) == 0:
+        raise AssertionError('No event was sent inside actions.py')
+
+    if len(m.call_args_list) > 1:
+        raise AssertionError('Too many events sent by action')
+
+    event_info.populate(m.call_args_list)
+
 @contextmanager
 def simulated_empty_cache() -> Generator[
         List[Tuple[str, Union[str, List[str]], str]], None, None]: