t/unit-tests: convert trailer test to use clar

t/unit-tests: convert trailer test to use clar

Adapt trailer test file to use clar testing framework by using clar
assertions where necessary. Split test into individual test functions
for clarity and maintainability. Each test case now has its own
function, making it easier to isolate failures and improve test
readability.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Seyi Kuforiji <kuforiji98@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>