test:set-check-box!

• (test:set-check-box! check-box state) -> void
      check-box : (union (instance check-box%) string)
      state : boolean

  Clears the check-box% item if state is #f, and sets it otherwise.

  If check-box is a string, this function searches for a check-box% with a label matching that string, otherwise it uses check-box itself.