tests: drivers: rtc: rtc_api: add config for alarm time mask

RTCs support a variety of combinations of alarm time fields, set
by the alarm time mask. Until now, alarm tests have selected
only the minute and hour fields, as these are always supported,
but some RTCs require setting every supported field when setting
the alarm time, while other RTCs don't support setting every field.

To support all RTCs in the test suite, a configuration has been
added which makes the alarm time mask configurable. Boards can now
define the specific alarm time mask they want to test within
their boards .conf files.

Additionally, the alarm tests have been refactored to not depend
on the time.h library to determine the struct rtc_time times to
set as these are constant, so they are now provided as const
structs instead.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>