If the underlying filesystem doesn't support atime or was mounted with the
noatime mount option, TestUtimesNanoAt fails when comparing atime:

--- FAIL: TestUtimesNanoAt (0.00s)
	syscall_linux_test.go:144: UtimesNanoAt: wrong atime: {1111 0}
	syscall_linux_test.go:147: UtimesNanoAt: wrong mtime: {3333 0}
FAIL
FAIL	golang.org/x/sys/unix	0.376s

Fix it by dropping the atime comparison, it's enough to verify UtimesNanoAt
working correctly by comparing mtime.

Fixes golang/go#26034

Change-Id: Id868f9d4c6a856a99ae930a46bfe91bd64ca8570
Reviewed-on: https://go-review.googlesource.com/120562
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>