avoid hanging test on bindValue problems (close db connection in tearDown)

tests/unit/framework/db/DatabaseTestCase.php

 <?php
 namespace yiiunit\framework\db;
 
+use yii\db\Connection;
 use yiiunit\TestCase as TestCase;
 
 abstract class DatabaseTestCase extends TestCase
 {
 	protected $database;
 	protected $driverName = 'mysql';
+	/**
+	 * @var Connection
+	 */
 	protected $db;
 
 	protected function setUp()
@@ -22,6 +26,13 @@ abstract class DatabaseTestCase extends TestCase
 		}
 	}
 
+	protected function tearDown()
+	{
+		if ($this->db) {
+			$this->db->close();
+		}
+	}
+
 	/**
 	 * @param bool $reset whether to clean up the test database
 	 * @param bool $open whether to open and populate test database