made the name of the testing environment configurable

everywhere i've ever worked has called their QA environment "testing", and their automated testing environment something else, whether it be "integration" or "phpunit" or whatever.

we had to rename our environment and process documents to "work" with laravel.

i've tried to make it configurable here, with a matching pull request in laravel/framework, so that people can call the testing environment whatever they want simply by modifying 1 line in start.php