t/: port helper/test-strcmp-offset.c to unit-tests/t-strcmp-offset.c

t/: port helper/test-strcmp-offset.c to unit-tests/t-strcmp-offset.c

In the recent codebase update (8bf6fbd (Merge branch
'js/doc-unit-tests', 2023-12-09)), a new unit testing framework was
merged, providing a standardized approach for testing C code. Prior to
this update, some unit tests relied on the test helper mechanism,
lacking a dedicated unit testing framework. It's more natural to perform
these unit tests using the new unit test framework.

Let's migrate the unit tests for strcmp-offset functionality from the
legacy approach using the test-tool command `test-tool strcmp-offset` in
helper/test-strcmp-offset.c to the new unit testing framework
(t/unit-tests/test-lib.h).

The migration involves refactoring the tests to utilize the testing
macros provided by the framework (TEST() and check_*()).

Helped-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Co-authored-by: Achu Luma <ach.lumap@gmail.com>
Signed-off-by: Achu Luma <ach.lumap@gmail.com>
Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>