    def _test_mapping_file_plain(self):
        def unichrs(s):
            return ''.join(chr(int(x, 16)) for x in s.split('+'))

        urt_wa = {}

        with self.open_mapping_file() as f:
            for line in f:
                if not line:
                    break
                data = line.split('#')[0].split()
                if len(data) != 2:
                    continue

                if data[0][:2] != '0x':
                    self.fail(f"Invalid line: {line!r}")
                csetch = bytes.fromhex(data[0][2:])
                if len(csetch) == 1 and 0x80 <= csetch[0]:
                    continue

                unich = unichrs(data[1])
                if ord(unich) == 0xfffd or unich in urt_wa:
                    continue
                urt_wa[unich] = csetch

                self._testpoint(csetch, unich)