Use require-dev in composer

Unless there is a really compelling reason not to use [require-dev][1] in the `composer.json` I would suggest removing the `composer.json` from the `test` directory and utilizing the `require-dev` property in the main composer file for the SimpleTest dependency.

You might want to add `--no-dev` to any build/deploy scripts you use in 'live' environments :wink: 

[1]: https://getcomposer.org/doc/04-schema.md#require-dev