net/http: fix race on postPendingDial test hook

net/http: fix race on postPendingDial test hook

The race occurs rarely, but by putting some delays and more reads/writes
of prePendingDial/postPendingDial in the handlePendingDial function I
could reproduce it.

Fixes #11136

Change-Id: I8da9e66c88fbda049eaaaaffa2717264ef327768
Reviewed-on: https://go-review.googlesource.com/11250
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>