Add test for closure in non-async function for needless_pass_by_ref_mut lint

Author: GuillaumeGomez

tests/ui/needless_pass_by_ref_mut.rs

@@ -270,6 +270,15 @@ pub async fn closure4(n: &mut usize) {
     })();
 }
 
+fn set_true(b: &mut bool) {
+    *b = true;
+}
+
+// Should not warn.
+fn true_setter(b: &mut bool) -> impl FnOnce() + '_ {
+    move || set_true(b)
+}
+
 fn main() {
     let mut u = 0;
     let mut v = vec![0];