Overwrite APP_ENV in PHPUnit

Important when you're using something like Docker where you inject the env variables directly into the environment rather than just pulling them from a .env file with a PHP dotenv component.

Make sure to use force="true" in your phpunit.xml.dist like:

<phpunit ...>
  <php>
    <env name="APP_ENV" value="test" force="true" />
    ...
  </php>
</phpunit>