Include Tests in Autoload

I added the tests namespace to the autoload so the test server could be used using autoload.

I've found two issues, with it not being available via autoload.

1. When including the file manually in a symfony2 application I got file could not be found on both travis-ci and drone.io. (Didn't get this issue locally)
2. It stops me from being fully compliant with PSR-2. In that it causes my test files to have a side effects.
See how I use it: https://github.com/GrahamCampbell/Laravel-Flysystem/blob/develop/composer.json#L37.