Replace ApiTestCase with PHPUnit_Framework_TestCase

- Move to `SMW\MediaWiki\Api` NS
- Replace `ApiTestCase` with `PHPUnit_Framework_TestCase` and remove ` ApiTestCase`
- Access to MediaWiki's `Api` during test is enabled through `SMW\Tests\Util\MwApiFactory`
- Add `SMW\Application` for external and internal instance access

```php
Application::getInstance()->getStore()
Application::getInstance()->newSerializerFactory()
Application::getInstance()->newJobFactory()
```