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);

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
+// }

test/input/reduce/unsafe_math.reduced.js

@@ -11,5 +11,12 @@ var a_1 = f0();
 
 console.log(b);
 // output: -19
+// 
 // minify: -4
-// options: {"compress":{"unsafe_math":true},"mangle":false}
+// 
+// options: {
+//   "compress": {
+//     "unsafe_math": true
+//   },
+//   "mangle": false
+// }