net/http/httptest: change Server to use http.Server.ConnState for accounting

net/http/httptest: change Server to use http.Server.ConnState for accounting

With this CL, httptest.Server now uses connection-level accounting of
outstanding requests instead of ServeHTTP-level accounting. This is
more robust and results in a non-racy shutdown.

This is much easier now that net/http.Server has the ConnState hook.

Fixes #12789
Fixes #12781

Change-Id: I098cf334a6494316acb66cd07df90766df41764b
Reviewed-on: https://go-review.googlesource.com/15151
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>