TestPendingConnsAfterErr only cared that things didn't deadlock, so 5
seconds is a sufficient timer. We don't need 100 milliseconds.

I was able to reproduce with a tiny (5 nanosecond) timeout value,
instead of 100 milliseconds. In the process of testing with -race and
a high -count= value, I noticed several data races and panics
(sendings on a closed channel) which are also fixed in this change.

Fixes #15684

Change-Id: Ib4605fcc0f296e658cb948352ed642b801cb578c
Reviewed-on: https://go-review.googlesource.com/24550Reviewed-by: Marko Tiikkaja <marko@joh.to>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>