Get rid of including phpunit classes from pear

We use hard code including PHPUnit Database package now. But it will cause the error if we use phpunit.phar or other phpunit package without pear.

If we type `php phpunit.phar` in Joomla dir, return this error:

```
Warning: require_once(PHPUnit/Extensions/Database/TestCase.php): failed to open stream: No such file or directory in D:\www\repo\joomladev\tests\unit\core\case\database.php on line 9

Fatal error: require_once(): Failed opening required 'PHPUnit/Extensions/Database/TestCase.php' (include_path='.;C:\bin\pear\pear') in D:\www\repo\joomladev\tests\unit\core\case\database.php on line 9PHP Warning:  require_once(PHPUnit/Extensions/Database/TestCase.php): failed to open stream: No such file or directory in D:\www\repo\joomladev\tests\unit\core\case\database.php on line 9

PHP Fatal error:  require_once(): Failed opening required 'PHPUnit/Extensions/Database/TestCase.php' (include_path='.;C:\bin\pear\pear') in D:\www\repo\joomladev\tests\unit\core\case\database.php on line 9

Process finished with exit code 255
```

So I do this change to get rid of hard code including PHPUnit Database from pear that developers can use their own phpunit package to test Joomla.