tests: allow testing if a path is truly a file or a directory

tests: allow testing if a path is truly a file or a directory

Add test_path_is_file_not_symlink(), test_path_is_dir_not_symlink()
and test_path_is_symlink(). Case of use for the first one
in test t/t3903-stash.sh to replace "test -f" because that function
explicitly want the file not to be a symlink.
Give more friendly error message.

Signed-off-by: COGONI Guillaume <cogoni.guillaume@gmail.com>
Co-authored-by: BRESSAT Jonathan <git.jonathan.bressat@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>