Rename Piwik_TestingEnvironment to TestingEnvironment and move the class to Piwik\Tests\Framework namespace. Remove manual require statements.

refs #5940 moved files into a subfolder framework, added autoloader to remove duplicated code to load autoload.php and to be able to register more autoloaders (eg for test files) on demand. This I got read of many includes that had to be updated all the time and that had to be updated all the time when moving iles

refs #6116