Tests: fixed h3_limit_req.t spurious failures. In the "reset stream - cancellation" test, HTTP/3 stream is closed without sending the request body when the request is waiting in the limit_req module, and this results in error 444. However, when the request is received with some minor delay due to system load, it is not delayed by limit_req, and the stream is closed during reading the request body, which results in error 400 instead, breaking the test. Fix is to introduce yet another request before the "reset stream" test, so the stream in question is always delayed by limit_req.