The `switch costume` block accepts special values like "next costume" and
"previous costume". If you create a costume with these names, these take
priority over the special values. However, the `switch backdrop` block
keeps these special values for values like "next backdrop", "previous
backdrop", "random backdrop". It is impossible to navigate to such a
backdrop by name via block. This commit also modifies tests to allow for
this.
BREAKING CHANGE: specially-named backdrops can now be navigated