Behat tests for Gitlab

[fjgarlin]

Integration of Behat tests on Gitlab. See it in action here: https://gitlab.com/fjgarlin/d9-lagoon/-/pipelines/285459468

The changes were minimal after achieving Githab actions.

I've also moved the test folder out as it was the same for all platforms.

[fjgarlin]

After this change, I think all platforms will be now D9 compatible and we will have coding standards, unit tests and behat tests running for all.

Doing this was also pretty useful in preparation for https://github.com/Lullabot/drupal9ci/issues/67

[juampynr]

Thanks @fjgarlin !