Add coverage for default_string and default_bytes in test_default_accessors

PiperOrigin-RevId: 588822948

rust/test/shared/accessors_test.rs

@@ -33,6 +33,8 @@ fn test_default_accessors() {
             default_bool(): eq(true),
         })
     );
+    assert_that!(msg.default_string(), eq("hello"));
+    assert_that!(msg.default_bytes(), eq("world".as_bytes()));
 }
 
 #[test]