improve --reduce-test (#3719)

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

test/input/reduce/label.reduced.js

@@ -0,0 +1,16 @@
+var o = this;
+
+for (var k in o) {
+    0;
+}
+
+var a;
+
+console.log(k);
+// output: a
+// 
+// minify: k
+// 
+// options: {
+//   "mangle": false
+// }