selftests: rtc: rework rtctest

Rework rtctest to use the test harness to better handle skipping tests
(e.g. when alarms are not available). Also, it now handles timeout so it
will not block expecting an alarm that never comes.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>