Add GitHub workflows for running tests

[GaryJones]

Because the package is still using only PHPUnit 7, I had to downgrade my local PHP version to PHP 7.4 just to get composer install to work.

I'd like to see this package using PHPUnit 9 (with the Yoast PHPUnit Polyfills package if still supporting PHP <= 7.2 which requires compatibility with PHPUnit 8 - see #198).

Only supporting PHPUnit 9 means we'd want a way to be able to run unit and integration tests on earlier versions of PHP and WP, so setting up a GitHub Workflow (example for unit tests and example for integration tests) would be beneficial.