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 <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>