LWT: Support for wide/unicode strings

Currently `BOOST_TEST_EQ(std::u16string{}, std::u16string{})` fails to compile. Similar for `wstring`, `u32string` and `u8string`.

Additionally support for `char8_t` comparison is missing.

While implementing this I'd suggest to try printing the ASCII subset of those directly similar to the `char` handling. The current differences in output are a bit confusing:
```
: test 's[0] == s2[1]' ('1' == '2') failed in function 'void test(const Ch *) [Ch = char]'
: test 's[0] == s2[1]' ('49' == '50') failed in function 'void test(const Ch *) [Ch = wchar_t]'
: test 's[0] == s2[1]' ('49' == '50') failed in function 'void test(const Ch *) [Ch = char16_t]'
: test 's[0] == s2[1]' ('49' == '50') failed in function 'void test(const Ch *) [Ch = char32_t]'
```