net/http: use runtime.AddCleanup instead of runtime.SetFinalizer

net/http: use runtime.AddCleanup instead of runtime.SetFinalizer

Replace the usage of runtime.SetFinalizer with runtime.AddCleanup in
tests.

Updates #70907

Change-Id: Idd3f1c07f6a7709352ca09948fbcb4a0ad9418bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/648655
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>