http2: fix nits in test Change-Id: Ib988bde8f8ff5719fd24bf63bd5ece648602dbd8 Reviewed-on: https://go-review.googlesource.com/23460 Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com> Reviewed-by: Andrew Gerrand <adg@golang.org>