tests/lib/timespec_util: Skip two tests by now

These two (sub)tests assume the system tick period divides evenly all
time interval values being tested, but this is not the case in general,
and when it is not, these tests fail.

Let's skip these 2 tests by now so we can release without a broken
CI, and do not block other development while the tests themselves are
being fixed.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>