t9160:modernize test path checking

t9160:modernize test path checking

Replace old-style path checks with Git's dedicated test helpers:
- test -f → test_path_is_file
- test -d → test_path_is_dir
- test -s → test_file_not_empty

Fix typos with the word "subsequent"

Found using: git grep "test -[efd]" t/

This improves test readability and provides better error messages
when path checks fail.

Signed-off-by: HodaSalim <hoda.s.salim@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>