t/unit-tests: convert strcmp-offset test to use clar test framework

t/unit-tests: convert strcmp-offset test to use clar test framework

Adapt strcmp-offset test script to clar framework by using clar
assertions where necessary. Introduce `test_strcmp_offset__empty()` to
verify `check_strcmp_offset()` behavior when both input strings are
empty. This ensures the function correctly handles edge cases and
returns expected values.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Seyi Kuforiji <kuforiji98@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>