http2: modernize TestTransportAllocationsAfterResponseBodyClose

This is a transport test. Use a testClientConn rather than a
Transport attached to a test server, for better control over
the frames sent to the test transport.

Drop one section of the test which pokes into the response
body's transportResponseBody type, as being too coupled to the
implementation internals.

Change-Id: I7bc7c7c756fd0c596424fab9a892dda8d9e89d1c
Reviewed-on: https://go-review.googlesource.com/c/net/+/701004
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>