go.net/context: adjust TestAlloc so it passes on windows
go.net/context: adjust TestAlloc so it passes on windows

Do not assume that a particular code path is selected
in WithTimeout.

Also Windows cannot wait for less then 15 millisecond,
so use that instead of 1 nanosecond in test. Otherwise
test takes too long while it looks like it is quick.

Fixes issue 8033.

LGTM=crawshaw
R=golang-codereviews, crawshaw
CC=golang-codereviews
https://codereview.appspot.com/103470044

ssor/go.net,fernandez14/go.net,polydawn-ports/go.net