net/http: start of making all relevant tests test both http1 and http2

net/http: start of making all relevant tests test both http1 and http2

This CL adds skipped failing tests, showing differences between HTTP/1
and HTTP/2 behavior. They'll be fixed in later commits.

Only a tiny fraction of the net/http tests have been split into their
"_h1" and "_h2" variants. That will also continue. (help welcome)

Updates #6891
Updates #13315
Updates #13316
Updates #13317

Change-Id: I16c3c381dbe267a3098fb266ab0d804c36473a64
Reviewed-on: https://go-review.googlesource.com/17046
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>