[release-branch.go1.15] net/http: speed up and deflake TestCancelRequestWhenSharingConnection

This test made many requests over the same connection for 10
seconds, trusting that this will exercise the request cancelation
race from #41600.

Change the test to exhibit the specific race in a targeted fashion
with only two requests.

Fixes #47534.
Updates #41600.
Updates #47016.

Change-Id: If99c9b9331ff645f6bb67fe9fb79b8aab8784710
Reviewed-on: https://go-review.googlesource.com/c/go/+/339594


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
(cherry picked from commit 6e738868)
Reviewed-on: https://go-review.googlesource.com/c/go/+/339829