kselftests: timers: Add adjtimex SETOFFSET validity tests

Add some simple tests to check both valid and invalid
offsets when using adjtimex's ADJ_SETOFFSET method.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Harald Hoyer <harald@redhat.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: David Herrmann <dh.herrmann@gmail.com>
Link: http://lkml.kernel.org/r/1453417415-19110-3-git-send-email-john.stultz@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>