Selenium code coverage setup

Files (`phpunit_coverage.php`, `prepent.php`, `append.php`) are located in [PHPUnit/Extensions/SeleniumCommon](https://github.com/sebastianbergmann/phpunit-selenium/tree/master/PHPUnit/Extensions/SeleniumCommon), not [PHPUnit/Extensions/SeleniumTestCase](https://github.com/sebastianbergmann/phpunit-selenium/tree/master/PHPUnit/Extensions/SeleniumTestCase)