selftests/timers: Add clock skew estimation test from timetest suite

This adds my clock skew estimation test from the timetest suite.
It measures the drift between CLOCK_MONOTONIC and CLOCK_MONOTONIC_RAW
and compares it with the current frequency value from adjtimex.

It sometimes can trigger false failures when ntpd isn't in a
steady state, but its a useful too when doing adjtimex testing.

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>