Add mixin test case
Add mixin test case

marijnh/tern,talyian/tern,albertinad/tern,emacsmirror/tern,albertinad/tern,messfairy/tern,talyian/tern,emacsmirror/tern,MarcelGerber/tern,mcanthony/tern,ternjs/tern,mcanthony/tern,mcanthony/tern,messfairy/tern,othree/tern,neoclide/tern,MarcelGerber/tern,talyian/tern,neoclide/tern,marijnh/tern,fileability/tern,fileability/tern,neoclide/tern,albertinad/tern,MarcelGerber/tern,marijnh/tern,messfairy/tern,ternjs/tern,fileability/tern,ternjs/tern,othree/tern,emacsmirror/tern,othree/tern