Add proper synchronisation to WebSocketTestSession (#2597)

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

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