t2400: replace 'test -[efd]' with 'test_path_is_*'

t2400: replace 'test -[efd]' with 'test_path_is_*'

'test_path_is_file', 'test_path_is_dir' and 'test_file_is_missing'
are test helpers used in Git's development, that emit useful
diagnostic information when they detect a failing condition, while
test -[efd] does not.

Replace the basic shell commands 'test -f', 'test -d' and 'test -e',
with these test helpers.

Co-authored-by: Isabella Caselli <icaselli@usp.br>
Signed-off-by: Isabella Caselli <icaselli@usp.br>
Signed-off-by: Rodrigo Michelassi <rodmichelassi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>