tests: rtc_api: Update y2k test to allow unknown values

Some RTCs don't support the tm_yday and/or tm_wday fields
of the struct rtc_time structure. These RTCs must set the
values of the fields to -1 if they are not supported, and
the test must be able to handle this correctly. This commit
adds an exception to the tm_yday and tm_wday fields allowing
them to be set to -1.

Signed-off-by: Bjarki Arge Andreasen <bjarkix123@gmail.com>