add test for bcrypt compatibility with old version

tests/bcrypt-compatibility-test.js

+#!/usr/bin/env node
+
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+require('./lib/test_env.js');
+
+const assert =
+require('assert'),
+vows = require('vows'),
+config = require('../lib/configuration.js'),
+bcrypt = require('bcrypt');
+
+var suite = vows.describe('bcrypt-compatibility');
+
+suite.addBatch({
+  "new bcrypt of password for given salt": {
+    topic: function () {
+      var salt = "$2a$04$rakQlaS/TyfjZmoVuRs9ku";
+      bcrypt.encrypt("Thisismypassword1!", salt, this.callback);
+    },
+    "should match old bcrypt": function (hash) {
+      assert.strictEqual(hash, '$2a$04$rakQlaS/TyfjZmoVuRs9kuQHFk2oShl8DNmVbxgSZyOE8Hzgk0One');
+    }
+  },
+  "get rounds of old hash should match new bcrypt": function () {
+    var hash = '$2a$04$rakQlaS/TyfjZmoVuRs9kuQHFk2oShl8DNmVbxgSZyOE8Hzgk0One';
+    assert.strictEqual(4, bcrypt.getRounds(hash));
+  }
+});
+
+if (process.argv[1] === __filename) suite.run();
+else suite.export(module);