add functional driver test case for mysqli driver

tests/Doctrine/Tests/DBAL/Functional/Driver/Mysqli/DriverTest.php

+<?php
+
+namespace Doctrine\Tests\DBAL\Functional\Driver\Mysqli;
+
+use Doctrine\DBAL\Driver\Mysqli\Driver;
+use Doctrine\Tests\DBAL\Functional\Driver\AbstractDriverTest;
+
+class DriverTest extends AbstractDriverTest
+{
+    protected function setUp()
+    {
+        if (! extension_loaded('mysqli')) {
+            $this->markTestSkipped('mysqli is not installed.');
+        }
+
+        parent::setUp();
+
+        if (! $this->_conn->getDriver() instanceof Driver) {
+            $this->markTestSkipped('MySQLi only test.');
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function createDriver()
+    {
+        return new Driver();
+    }
+}