timers, kselftest: Add 'adjtick' test to validate adjtimex() tick adjustments

Recently a kernel side NTP bug was fixed via the following commit:

  2619d7e9 ("time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()")

When the bug was reported it was difficult to detect, except by
tweaking the adjtimex tick value, and noticing how quickly the
adjustment took:

	https://lkml.org/lkml/2015/9/1/488



Thus this patch introduces a new test which manipulates the
adjtimex tick value and validates that the results are what we
expect.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Nuno Gonçalves <nunojpg@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1444094217-20258-1-git-send-email-john.stultz@linaro.org


[ Tidied up the code and the changelog a bit. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>