Solution for some dummies without Iconv

This solution tries to preserve original string unchanged and instead make `Regexp` in the corresponding encoding, but resorts to `UTF-8` for dummy encodings (Regexp refuses to be built with dummy enc).

Test skips over a subset of dummy encodings that Ruby does not even have a converter  for UTF-8 (i.e. test can't even create initial string in that encoding). It would be possible to use `Iconv` to create the strings and then compare  as other dummies, but I wanted to keep it simple.