Make Process.WaitForExitAsync wait for redirected output reads (#42585)

Make Process.WaitForExitAsync wait for redirected output reads (#42585)

* Make Process.WaitForExitAsync wait for output reads

Addresses an issue where Process.WaitForExitAsync
doesn't wait for background redirected output reads,
a behaviour which diverges from the sync method
equivalent. Fixes #42556.

* address feedback

* pass cancellation token to AsyncStreamReader waiter

* address feedback

* use tcs.TrySetResult

* Update src/libraries/Common/src/System/Threading/Tasks/TaskTimeoutExtensions.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>