Add the Silverstripe Github action for automated unit testing

[edwilde]

Resolves #28

Adding the standard Silverstripe Github action to automatically run the tests against supported versions of php.

The tests will be run:

• When any code is pushed to the repo
• Monthly at midnight on the first day of the month - to catch external changes

In addition, I also upgraded phpunit to 9.6, with test changes copied from the SS5 pull request, with SS4 compatibility added.