tests/drivers/rtc: Add unit tests for RTC devices

This test suite adds tests for the following:

- Setting and getting time
- Validating time is incrementing correctly
- Validating behavior of alarms with callback disabled
- Validating behavior of alarms with callback enabled
- Validating update callback

The test suite uses the devicetree  alias rtc to find
the device to test.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>