improve --reduce-test (#3719)

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

test/input/reduce/unsafe_math.reduced.js

@@ -0,0 +1,22 @@
+var b = 0;
+
+function f0() {
+    var expr2 = (0 - 1 - .1 - .1).toString();
+    for (var key2 in expr2) {
+        --b;
+    }
+}
+
+var a_1 = f0();
+
+console.log(b);
+// output: -19
+// 
+// minify: -4
+// 
+// options: {
+//   "compress": {
+//     "unsafe_math": true
+//   },
+//   "mangle": false
+// }