tools, update rtctest.c to verify passage of time

rtctest.c checks to see if PIE is functioning by testing if 20 interrupts occur
at rates from 2HZ to 64HZ.  While this check is good, it does not check to
see if the correct amount of time has actually passed.  This misses
situations where the RTC may be operating at a higher or lower frequency
than expected.

This patch introduces a simple check to verify if the time passed is
less than 10% of what was programmed into the RTC.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Cc: corbet@lwn.net
Cc: rtc-linux@googlegroups.com
Cc: linux-doc@vger.kernel.org
Cc: a.zummo@towertech.it
Cc: prarit@redhat.com
Cc: john.stultz@linaro.org
Cc: shuahkh@osg.samsung.com
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>