t3600: use helpers to replace test -d/f/e/s <path>

Take advantage of helper functions test_path_is_dir(),
test_path_is_missing(), etc. to replace `test -d|f|e|s` since the
functions make the code more readable and have better error
messages.

Signed-off-by: Rohit Ashiwal <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: r1walz