t7601: use "test_path_is_file" etc. instead of "test -f"

t7601: use "test_path_is_file" etc. instead of "test -f"

Some tests in t7601 use "test -f" and "test ! -f" to see if a path
exists or is missing.

Use test_path_is_file and test_path_is_missing helper functions to
clarify these tests a bit better. This especially matters for the
"missing" case because "test ! -f F" will be happy if "F" exists as a
directory, but the intent of the test is that "F" should not exist, even
as a directory. The updated code expresses this better.

Signed-off-by: Dorcas AnonoLitunya <anonolitunya@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>