This makes TestTransportResponseCloseRace much faster and no longer
flaky.

In the process it also cleans up test hooks in net/http which were
inconsistent and scattered.

Change-Id: Ifd0b11dbc7e8915c24eb5bdc36731ed6751dd7ec
Reviewed-on: https://go-review.googlesource.com/17316Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>