Check requirements before "beforeClass" calls in a TestSuite

This PR aims to fix fatal errors like "Class 'ClassFromNonExistingExtension' not found ..." which is generated for the following test case: 

```php
<?php

/**
 * @requires extension nonExistingExtension
 */
class FooTest extends PHPUnit_Framework_TestCase
{
    public static function setUpBeforeClass()
    {
        // call a function from a non-existing extension
    }
}
```