Fix the ping test by adding a delay to the write operation

[Matthias247]

The TestPing unit test failed on my machine since it always reported a rtt of 0. I guess that is since the Go timers are not very accurate on windows and writing through pipes is really fast.

I fixed that now by introducing a delay/sleep to the write.
It's a little hacky since I grab and modify the connection from the session which would be unsafe if it sends anything while not externally called. But I think it doesn't do that as long as there are not streams, pings or keepalives running and works for that test.

The other alternative is to introduce a general delay option to pipeConn which I did first. But there unit tests need to be modified in lots of places to set that new parameter.

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• evanphx
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[evanphx]

This is a good fix for the tests, @Matthias247 I realize this is quite old, did you want to sign the CLA so I can merge this? Thanks!