added simple unit test for CacheSession

tests/unit/framework/web/CacheSessionTest.php

+<?php
+
+namespace yiiunit\framework\web;
+
+use Yii;
+use yii\caching\FileCache;
+use yii\web\CacheSession;
+
+/**
+ * @group web
+ */
+class CacheSessionTest extends \yiiunit\TestCase
+{
+	protected function setUp()
+	{
+		parent::setUp();
+		$this->mockApplication();
+		Yii::$app->setComponent('cache', new FileCache());
+	}
+
+	public function testCreate()
+	{
+		$session = new CacheSession();
+
+		$session->writeSession('test', 'sessionData');
+		$this->assertEquals('sessionData', $session->readSession('test'));
+	}
+}