Add proper synchronisation to WebSocketTestSession (#2597)

* Add proper synchronisation to WebSocketTestSession
`anyio.sleep(0)` is often used as a way to yield to another task.
However, depending on event loop implememtation it is not guaranteed to
actually do so in a timely manner.
This commit alters this behaviour in _asgi_receive by using
`anyio.Event`s as a simple synchronisation primitive, dramatically
speeding up the session depending on underlying system/implementation.

* Fix mypy type errors
Jinja 3.1.4 slightly changed the argument types of FileSystemLoader.

* Formatting

---------

Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>

starlette/testclient.py

@@ -160,7 +160,8 @@ async def run_app(tg: anyio.abc.TaskGroup) -> None:
 
     async def _asgi_receive(self) -> Message:
         while self._receive_queue.empty():
-            await anyio.sleep(0)
+            self._queue_event = anyio.Event()
+            await self._queue_event.wait()
         return self._receive_queue.get()
 
     async def _asgi_send(self, message: Message) -> None:
@@ -189,6 +190,8 @@ def _raise_on_close(self, message: Message) -> None:
 
     def send(self, message: Message) -> None:
         self._receive_queue.put(message)
+        if hasattr(self, "_queue_event"):
+            self.portal.start_task_soon(self._queue_event.set)
 
     def send_text(self, data: str) -> None:
         self.send({"type": "websocket.receive", "text": data})