Add acceptance testing framework

We should set up acceptance testing with the goals of:

a) avoiding back compat breakages
b) ensuring stable functionality across most popular configurations and documented usages
c) preventing regressions for fixed bugs

This should be done using a GH action. Explore tools including wp-scripts e2e tools, wp-env, codeception.

Because the multisite cross network redirect feature relies on JS to submit a form the testing tool will need to run a browser (or browser-like environment that can execute JS) for tests.

The scope of this ticket is not to add any tests.

Included in the scope:

* create the setup to run them against pull requests
* be able to be run locally
* document how to run tests