Enable testing multibyte named backreferences in v5.
Enable testing multibyte named backreferences in v5.

I will test this with lesser versions of Oniguruma to see if they
also work given the recent changes. If not then I think I need to
find a way to detect multibyte named backreferences in v2/4 and
raise an exception.

geoffgarside/oniguruma,geoffgarside/oniguruma