add benchmark & JetStream tests

- `test/benchmark.js` measures performance
- `test/jetstream.js` verifies correctness
- configurable mangle/compress/output options

closes #1479

test/benchmark.js

@@ -0,0 +1,49 @@
+#! /usr/bin/env node
+// -*- js -*-
+
+"use strict";
+
+var createHash = require("crypto").createHash;
+var fork = require("child_process").fork;
+var args = process.argv.slice(2);
+if (!args.length) {
+    args.push("-mc", "warnings=false");
+}
+args.push("--stats");
+var urls = [
+    "https://code.jquery.com/jquery-3.1.1.js",
+    "https://code.angularjs.org/1.6.1/angular.js",
+    "https://cdnjs.cloudflare.com/ajax/libs/mathjs/3.9.0/math.js",
+    "https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.js",
+    "https://unpkg.com/react@15.3.2/dist/react.js",
+    "http://builds.emberjs.com/tags/v2.11.0/ember.prod.js",
+    "https://cdn.jsdelivr.net/lodash/4.17.4/lodash.js",
+    "https://cdnjs.cloudflare.com/ajax/libs/d3/4.5.0/d3.js",
+];
+var results = {};
+var remaining = 2 * urls.length;
+function done() {
+    if (!--remaining) {
+        urls.forEach(function(url) {
+            console.log();
+            console.log(url);
+            console.log(results[url].time);
+            console.log("SHA1:", results[url].sha1);
+        });
+    }
+}
+urls.forEach(function(url) {
+    results[url] = { time: "" };
+    require(url.slice(0, url.indexOf(":"))).get(url, function(res) {
+        var uglifyjs = fork("bin/uglifyjs", args, { silent: true });
+        res.pipe(uglifyjs.stdin);
+        uglifyjs.stdout.pipe(createHash("sha1")).on("data", function(data) {
+            results[url].sha1 = data.toString("hex");
+            done();
+        });
+        uglifyjs.stderr.setEncoding("utf8");
+        uglifyjs.stderr.on("data", function(data) {
+            results[url].time += data;
+        }).on("end", done)
+    });
+});