net/http: speed up and deflake TestCancelRequestWhenSharingConnection
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.

Updates #41600.
Updates #47016.

Change-Id: If99c9b9331ff645f6bb67fe9fb79b8aab8784710
Reviewed-on: https://go-review.googlesource.com/c/go/+/339594
Trust: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

golang/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go