net/http: add more tests of transport connection pool
Add a variety of addtional tests exercising client connection pooling,
in particular HTTP/2 connection behavior.
Change-Id: I7609d36db5865f1b95c903cfadb0c3233e046c09
Reviewed-on: https://go-review.googlesource.com/c/go/+/615896 Reviewed-by: Nicholas Husin <husin@google.com> Reviewed-by: Nicholas Husin <nsh@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>