net/http: deflake TestCancelRequestWhenSharingConnection
net/http: deflake TestCancelRequestWhenSharingConnection

The test sleeps for 1 millisecond to give the cancellation a moment
to take effect. This is flaky because the request can finish before
the cancellation of the context is seen. It's easy to verify by adding

    time.Sleep(2*time.Millisecond)

after https://github.com/golang/go/blob/0a6c4c87404ecb018faf002919e5d5db04c69ee2/src/net/http/transport.go#L2619.
With this modification, the test fails about 5 times out of 10 runs.

The fix is easy. We just need to block the handler of the second
request until this request is cancelled. I have verify that the
updated test can uncover the issue fixed by CL 257818.

Fixes #55226.

Change-Id: I81575beef1a920a2ffaa5c6a5ca70a4008bd5f94
GitHub-Last-Rev: 99cb1c2eaed7839394adbb6bbcd4950cd4bfb6f3
GitHub-Pull-Request: golang/go#56500
Reviewed-on: https://go-review.googlesource.com/c/go/+/446676
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go