Force colors when running composer test

[bramus]

When running composer test it does not use coloring, even though phpunit.xml.dist has colors set to true.

(Sidenote: using the default macOS Terminal.app here, with a custom prompt)

This PR adjusts the composer test script so that it enforces the use of colors wit PHPUnit.

Here's the result (before and after):

[brendt]

Thanks!