net/http: TestCancelRequestWhenSharingConnection is flaky on release-branch.go1.15 branch

[cagedmantis]

2505e26/windows-amd64-longtest
1035bb9/windows-amd64-longtest
b35c38a/windows-amd64-longtest
636a6d14/windows-amd64-longtest

May be related to ##42942

CC CC @alexbrainman @fraenkel @neild @bradfitz @golang/release

[fraenkel]

@cagedmantis I think this is the wrong test. While the error occurs on TestCancelRquestWhenSharing after a few seconds, the moire obvious one is TestMissingStatusNoPanice which has been stuck for 14m.

goroutine 17971 [chan receive, 14 minutes]:
net/http_test.TestMissingStatusNoPanic(0xc00060e000)
	C:/workdir/go/src/net/http/transport_test.go:5320 +0x450