t9146: replace test -d/-e/-f with appropriate test_path_is_* function

t9146: replace test -d/-e/-f with appropriate test_path_is_* function

The helper functions test_path_is_* provide better debugging
information than test -d/-e/-f.

Replace "if ! test -d then <error message>" and "test -d" with
"test_path_is_dir" at places where we check for existent directories.

Replace "test -f" with "test_path_is_file" at places where we check
for existent files.

Replace "test ! -e" and "if test -d then <error message>" with
"test_path_is_missing" where we check for non-existent directories.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>