http2: speed up TestTransportFlowControl

Rewrite the slowest test in the http2 package.

This test was added in CL 23812 to verify that a Transport does not
provide flow control window updates until the user consumes data
from a stream.

Rewrite the test to use the more modern testClientConn, which permits
precise examination of when the transport sends WINDOW_UPDATE frames.

Change-Id: Ibcde492549cad6363ce0e1a5ba169da7a4427d85
Reviewed-on: https://go-review.googlesource.com/c/net/+/700923
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>