Add functional / behavioral tests

[jazzsequence]

Any code running on Pantheon sites should be validated to continue to work on Pantheon sites. This can be done via behavioral tests (e.g. Behat) or more end-to-end, Javascript-based tests (Playwright, Cyprus, etc).

Example Behat test suite: https://github.com/pantheon-systems/pantheon-advanced-page-cache/tree/release/tests/behat
Example Playwright tests: https://github.com/pantheon-systems/wordpress-composer-managed/blob/default/.github/tests/wpcm.spec.ts [Test configuration]