t/pack-refs-tests: use test_path_is_missing

t/pack-refs-tests: use test_path_is_missing

The pack-refs tests previously used raw 'test -f' and 'test -e' checks
with negation. Update them to use Git's standard helper function
test_path_is_missing for consistency and clearer failure reporting.

As suggested in review, replaced the negated 'test_path_exists' with
test_path_is_missing to better reflect the expected absence of paths.

Signed-off-by: Ritesh Singh Jadoun <riteshjd75@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>