net/http: TestTransportCancelBeforeResponseHeaders is flaky

[bradfitz]

I'm going to disable this test for now, as it's flaky with:

--- FAIL: TestTransportCancelBeforeResponseHeaders (0.00s)
    transport_test.go:1592: RoundTrip error = io: read/write on closed pipe; want ExportErrRequestCanceled
FAIL

Fix and re-enable later.

[gopherbot]

CL https://golang.org/cl/12731 mentions this issue.

[gopherbot]

CL https://golang.org/cl/18287 mentions this issue.