added array fixture class

Added `ArrayFixture` class file, related with #4875 . @qiangxue you maybe may want to perform some code checks due some duplication with [`BaseActiveFixture`](https://github.com/yiisoft/yii2/blob/master/framework/test/BaseActiveFixture.php) and make it extend `ArrayFixture` , but i am not sure about this one , also it already extends `DbFixture`