unix: in TestSelect, only error for an early wakeup on Linux
unix: in TestSelect, only error for an early wakeup on Linux

On Linux, 'man 2 select' explicitly lists the conditions under which
select may return before the timeout interval. Most other platforms
make no such guarantee, so do not test for it on those platforms.

Fixes golang/go#36409

Change-Id: I194a34af3132a7db7fc186dec9de67fa6dd0bfae
Reviewed-on: https://go-review.googlesource.com/c/sys/+/363455
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/sys,golang/sys,golang/sys