Refactor how filtering works

Right now, File_Iterator is used to crawl the filesystem for files that match the criteria specified in the (PHPUnit) configuration resulting in a list (array) of files that are to be included in the code coverage report. This crawling is responsible for a slowdown in the startup of PHPUnit.

It would be better to store the information ("all *.php files in directory src) instead and match the filenames against that instead of a list generated through crawling.