net/http: TestCancelRequestWhenSharingConnection can cause port exhaustion, leading to frequent test failures with "Only one usage of each socket address" on windows-amd64-longtest [1.15 backport]

[gopherbot]

@neild requested issue #47016 to be considered for backport to the next 1.15 minor release.

@gopherbot please open backport issues.

[gopherbot]

Change https://golang.org/cl/339829 mentions this issue: [release-branch.go1.15] net/http: speed up and deflake TestCancelRequestWhenSharingConnection

[gopherbot]

Closed by merging e90f402 to release-branch.go1.15.