t5520: replace test -f with test-lib functions
Although `test -f` has the same functionality as test_path_is_file(), in
the case where test_path_is_file() fails, we get much better debugging
information.
Replace `test -f` with test_path_is_file() so that future developers
will have a better experience debugging these test cases.
Also, in the case of `! test -f`, not only should that path not be a
file, it shouldn't exist at all so replace it with
test_path_is_missing().
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>