tests/drivers/rtc: Add rtc_api_helpers test suite

This test suite validates the RTC API helper
functions, like currently, and exclusively, the
rtc_time_to_tm() helper function.

It also validates that the struct rtc_time is
member to member compatible with the struct tm
from the tm_sec to and including the tm_isdts
member.

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