Avoid request body rate timeout in flaky test

[halter73]

GET_GracefulServerShutdown_AbortRequestsAfterHostTimeout is testing the behavior of shutdown timeouts, not rate timeouts.

[6.098s] [Microsoft.AspNetCore.Server.Kestrel.BadRequests] [Debug] Connection id "0HMF7DR9L1H02", Request id "null": the request timed out because it was not sent by the client at a minimum of 240 bytes/second.

[7.351s] [Microsoft.AspNetCore.Server.Kestrel] [Warning] As of "02/03/2022 23:42:38 +00:00", the heartbeat has been running for "00:00:01.2496930" which is longer than "00:00:01". This could be caused by thread pool starvation.

Even though the Http3ReuestsTests sets the ShutdownTimeout to 1 second instead of the 5 second default, it looks like timer didn't fire fast enough due to likely threadpool starvation. If the threadpool starvation is bad enough, even this won't help, but this should give more time for the threadpool to work the queue of work items.

Addresses #39985