add test for --reduce-test (#3712)

test/input/reduce/output.js

@@ -0,0 +1,15 @@
+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}