net/http: speed up and deflake TestCancelRequestWhenSharingConnection

[ferrmin]

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

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

Updates golang#41600.
Updates golang#47016.

Change-Id: If99c9b9331ff645f6bb67fe9fb79b8aab8784710
Reviewed-on: https://go-review.googlesource.com/c/go/+/339594
Trust: Damien Neil [email protected]
Run-TryBot: Damien Neil [email protected]
TryBot-Result: Go Bot [email protected]
Reviewed-by: Heschi Kreinick [email protected]

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