selftests/timers: Add change_skew test from timetest suite

selftests/timers: Add change_skew test from timetest suite

This patch adds the change_skew test which validates the
adjtimex freq can be set to various values and then using
the inconsistency-check, raw_skew, and nanosleep tests
ensures time behaves properly.

Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Tested-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>