improve --reduce-test (#3742)

- ignore difference in error messages
- improve readability on trailing whitespace differences
- improve performance & quality via `console.log()` insertions

test/input/reduce/label.reduced.js

@@ -1,8 +1,7 @@
+// (beautified)
 var o = this;
 
-for (var k in o) {
-    0;
-}
+for (var k in o) {}
 
 var a;