improve --reduce-test (#3719)

- cover missing cases when eliminating unreferenced labels
- format multi-line outputs correctly

test/input/reduce/label.js

@@ -0,0 +1,9 @@
+var o = this;
+
+for (var k in o) L17060: {
+    a++;
+}
+
+var a;
+
+console.log(k);