Updates http2 to x/net/http2 git rev 8fd7f25 for:

    http2: clear WriteTimeout in Server
    https://golang.org/cl/34724

And un-skip the new test. (The new test is a slow test, anyway, so
won't affect builders or all.bash, but I verified it now passes.)

Updates #18437

Change-Id: Ia91ae702edfd23747a9d6b61da284a5a957bfed3
Reviewed-on: https://go-review.googlesource.com/34729
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Kale B <kale@lemnisys.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>