Change test configuration for D8

The D8 automated tests are currently set up to run against PHP 5.6. Drupal 8 requires PHP 7, so the tests aren't going to work.

I suggest setting the main automated tests for D8 to use PHP 7.4 and MySQL 5.7 or even MySQL 8, that way they're running a compatible version and will be faster than older releases.

To ensure compatibility with PHP 7.0 an extra automated test could be set up to run on commit, that way the module can be tested against the older version but the day-to-day testing would be handled by a faster setup.