Test for correct timeout behavior of Select and Pselect

Updates golang/go#22246

Change-Id: I86d1804c6ddf5072e93f3ef4f359198e732fae94
Reviewed-on: https://go-review.googlesource.com/84955
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>