net/http: TestClientTimeoutCancel is flaky

From the `linux-ppc64le-power9osu` (https://build.golang.org/log/bc146619a7844f5eadaff5632418865af83dacf4):

```
--- FAIL: TestClientTimeoutCancel (0.00s)
    client_test.go:1353: error = read tcp 127.0.0.1:42466->127.0.0.1:40975: use of closed network connection; want errRequestCanceled
FAIL
FAIL	net/http	7.264s
```

CC @laboger @bradfitz 