net/http: run more tests in http2 mode

net/http: run more tests in http2 mode

Failing ones are marked skipped.

Fixes #13543 (was just a test issue)
Updates #13555 (to be fixed later)
Updates #13556 (to be fixed later)
Updates #13557 (to be fixed later)
Fixes bug in golang.org/cl/17428 (http1 now uses HTTP status 431, not 413)

Change-Id: I8f7431fee35f2fc081cfe2c232ae75a00800a60b
Reviewed-on: https://go-review.googlesource.com/17683
Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Burcu Dogan <jbd@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>