twister: add setting skip reason

When testcase written in ztest is skipped by ztest_test_skip() function,
then "SKIP" information is printed in output data. This "SKIP" result is
set as result of performed testcase, but the information about reason is
still "Unknown". This patch fix this problem and if all test scenario
has state "passed" (so testcases have result "PASS" or "SKIP"), reason
is still set as "Unknown" and if any of testcase has result "SKIP" then
set reason as "ztest skip".

Fixes #42414

Signed-off-by: Piotr Golyzniak <piotr.golyzniak@nordicsemi.no>