Add tests for String#rindex of mruby-string-utf8.

Note: this tests aren't executed  because there is not mruby-string-utf8  in default.gembox.
I tested mruby-string-utf8 in the following config:
build_config.rb
```ruby
  # include the default GEMs
  #conf.gembox 'default'
  conf.gembox 'full-core'
```