t5315: use test_path_is_file for loose-object check

t5315: use test_path_is_file for loose-object check

Use test_path_is_file instead of test -f when checking that the
loose object was written to the expected path.

This uses Git's path-checking helper, which provides more specific
failure output than a raw test -f check.

Signed-off-by: Bilal El Khatabi <elkhatabibilal@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>