Dome some 'dumb' testing - and I hope this helps understand why it is failing:

With the the last two func() calls commented out, the function passes:

    def test_output_string(self):
        stdscr = self.stdscr
        encoding = stdscr.encoding
        # addstr()/insstr()
        for func in [stdscr.addstr, stdscr.insstr]:
            with self.subTest(func.__qualname__):
                stdscr.move(0, 0)
                func('abcd')
                func(b'abcd')
                s = 'à▒^▒ç▒^▒'
                try:
                    func(s)
                except UnicodeEncodeError:
                    self.assertRaises(UnicodeEncodeError, s.encode, encoding)
                func('abcd', curses.A_BOLD)
                # func(1, 2, 'abcd')
                # func(2, 3, 'abcd', curses.A_BOLD)