http2: add transport API tests

Add a collection of tests which specifically exercise the user-facing
client APIs in this package.

Many of these tests are somewhat redundant with existing tests in
the package. Their intent is to ensure consistency between various
ways users may access HTTP/2 features:

  - Using a x/net/http2.Transport directly.
  - Using a net/http.Transport configured with ConfigureTransport.

and:

  - Using the HTTP/2 implementation in x/net/http2.
  - Using the HTTP/2 implementation in net/http, wrapped by x/net/http2.
    (A case to be implemented in future CLs.)

For #78508

Change-Id: I228cc1980461ca5c5b4a080bb966450b6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/762320
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>