tests: add the _ as expr test for cargo fix

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Author: vincenzopalazzo

tests/ui/macros/expr_2021_cargo_fix_edition.rs

@@ -20,5 +20,6 @@ macro_rules! test {
 
 fn main() {
     m!(());
+    m!(_);
     test!(expr);
 }