refactored database tests

- changed test class naming for easier copying and searching of classes
- made base tests abstract and made MySQL an equal citisen in the test
  hierarchie
- added missing db specific test classes

tests/framework/db/mysql/BatchQueryResultTest.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace yiiunit\framework\db\mysql;
+
+/**
+ * @group db
+ * @group mysql
+ */
+class BatchQueryResultTest extends \yiiunit\framework\db\BatchQueryResultTest
+{
+    public $driverName = 'mysql';
+}