unix: add test for UtimesNanoAt on Linux
unix: add test for UtimesNanoAt on Linux

Add a test for UtimesNanoAt which makes sure the timestamp of a symlink
is updated.

Change-Id: I819e43db1d390fb37f011b34e58ff9d3d06595f1
Reviewed-on: https://go-review.googlesource.com/72377
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys,golang/sys,multiplay/golang-sys