http2: support testing via net/http.Transport.RoundTrip

The testTransport test harness provides support for
testing Transport.RoundTrip.

Rework testTransport so it can use RoundTrip on either
x/net/http2.Transport or net/http.Transport.

This change will let us write tests which verify
consistent behavior when using the http2 package's
Transport directly or via the net/http Transport.

For #78508

Change-Id: I7bc34973c0cc9a26b1f0cff56d4fdf4f6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/762300
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
Auto-Submit: Damien Neil <dneil@google.com>