hir_ty: add coverage testing for autoderef_visibility_method test

crates/hir_ty/src/method_resolution.rs

@@ -687,6 +687,7 @@ fn is_valid_candidate(
             }
             if let Some(from_module) = visible_from_module {
                 if !db.fn_visibility(m).is_visible_from(db.upcast(), from_module) {
+                    cov_mark::hit!(autoderef_candidate_not_visible);
                     return false;
                 }
             }

crates/hir_ty/src/tests/method_resolution.rs

@@ -1227,6 +1227,7 @@ mod b {
 
 #[test]
 fn autoderef_visibility_method() {
+    cov_mark::check!(autoderef_candidate_not_visible);
     check_infer(
         r#"
 #[lang = "deref"]