Add unittest

Add unittest

[  155s] + ./derive_more
[  155s]
[  155s] running 4 tests
[  155s] test display::regex_maybe_placeholder_spec::parses_placeholders_and_omits_escaped ... ok
[  155s] test display::placeholder_parse_fmt_string_spec::indicates_position_and_trait_name_for_each_fmt_placeholder ... ok
[  155s] test display::regex_placeholder_format_spec::detects_arg ... ok
[  155s] test display::regex_placeholder_format_spec::detects_type ... ok
[  155s]
[  155s] test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s