def test_cat_nonexistent(self):
        """test 'cat <some file which does not exist>'."""
        output = self.run_command("cat invalid.txt", exitcode=1)
        self.assertIn("cat: ", output)
        self.assertIn("No such file or directory: ", output)
        self.assertIn("invalid.txt", output)
test 'cat <some file which does not exist>'.
tests/cat/test_cat.py