Updates x/net/http2 to git rev 6bdd4be4 for CL 23526:

  http2: GotFirstResponseByte hook should only fire once

Also updated the trace hooks test to verify that all trace hooks are called
exactly once except ConnectStart/End, which may be called multiple times (due
to happy-eyeballs).

Fixes #15777

Change-Id: Iea5c64eb322b58be27f9ff863b3a6f90e996fa9b
Reviewed-on: https://go-review.googlesource.com/23527Reviewed-by: Andrew Gerrand <adg@golang.org>