Add test for closure_binder

Co-authored-by: Lukas Wirth <me@lukaswirth.dev>

Author: aibaars

crates/parser/src/grammar/expressions/atom.rs

@@ -562,6 +562,8 @@ fn closure_expr(p: &mut Parser<'_>) -> CompletedMarker {
 
     let m = p.start();
 
+    // test closure_binder
+    // fn main() { for<'a> || (); }
     if p.at(T![for]) {
         let b = p.start();
         types::for_binder(p);