Fix gtk_text_iter_forward_to_tag_toggle() for end iter

Fix gtk_text_iter_forward_to_tag_toggle() for end iter

The function must return TRUE only if there is a tag toggle _after_ the
iter, not _at_ the iter. So for the end iter, the function must always
return FALSE.

Add also unit tests for gtk_text_iter_forward_to_tag_toggle().

https://bugzilla.gnome.org/show_bug.cgi?id=691266