spec: Factor MockStorageApi out of webstorage.spec.js

This is useful elsewhere.

spec/MockStorageApi.js

@@ -0,0 +1,54 @@
+/*
+Copyright 2015, 2016 OpenMarket Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * A mock implementation of the webstorage api
+ * @constructor
+ */
+function MockStorageApi() {
+    this.data = {};
+    this.keys = [];
+    this.length = 0;
+}
+
+MockStorageApi.prototype = {
+    setItem: function(k, v) {
+        this.data[k] = v;
+        this._recalc();
+    },
+    getItem: function(k) {
+        return this.data[k] || null;
+    },
+    removeItem: function(k) {
+        delete this.data[k];
+        this._recalc();
+    },
+    key: function(index) {
+        return this.keys[index];
+    },
+    _recalc: function() {
+        var keys = [];
+        for (var k in this.data) {
+            if (!this.data.hasOwnProperty(k)) { continue; }
+            keys.push(k);
+        }
+        this.keys = keys;
+        this.length = keys.length;
+    }
+};
+
+/** */
+module.exports = MockStorageApi;

spec/unit/webstorage.spec.js

@@ -5,36 +5,7 @@ var Room = sdk.Room;
 var User = sdk.User;
 var utils = require("../test-utils");
 
-function MockStorageApi() {
-    this.data = {};
-    this.keys = [];
-    this.length = 0;
-}
-MockStorageApi.prototype = {
-    setItem: function(k, v) {
-        this.data[k] = v;
-        this._recalc();
-    },
-    getItem: function(k) {
-        return this.data[k] || null;
-    },
-    removeItem: function(k) {
-        delete this.data[k];
-        this._recalc();
-    },
-    key: function(index) {
-        return this.keys[index];
-    },
-    _recalc: function() {
-        var keys = [];
-        for (var k in this.data) {
-            if (!this.data.hasOwnProperty(k)) { continue; }
-            keys.push(k);
-        }
-        this.keys = keys;
-        this.length = keys.length;
-    }
-};
+var MockStorageApi = require("../MockStorageApi");
 
 describe("WebStorageStore", function() {
     var store, room;