net/http: disable new flaky TestTransportCancelBeforeResponseHeaders test

I'll rewrite this later. It's apparently dependent on scheduling order.
The earlier fix in git rev 9d56c181 seems fine, though.

Update #11894

Change-Id: I7c150918af4be079c262a5f2933ef4639cc535ef
Reviewed-on: https://go-review.googlesource.com/12731


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>