Extract PHPUnit compatibility into its own package

[Baptouuuu]

Everything in the Innmind\BlackBox\PHPUnit namespace should be extracted into a innmind/black-box-package.

This package should be required as a dependency until the next major release to avoid BC breaks.

This will allow in the future to not install PHPUnit for users who only rely on the BlackBox runner.

Details

This requires a circular dependency. To avoid any problems:

• the sub-package will require this one as a dev dependency
• this package will require the sub-package

This way it's only a loose circular dependency.

And when the next major version of this package is released, the sub-package will move the dependency on this one as a required one.