New testing level: php integration tests with configurable plugins

[diosmosis]

To ensure compatibility with other wordpress plugins and prevent regressions in with compatibility, an integration testing system should be created. These tests would:

• use docker or wp-env to automatically setup an environment with the required plugins (latest version always)
• run PHP tests that include different parts of matomo-for-wordpress (for example, AjaxTracker) and and test different things, with all required plugins loaded

This issue would be complete after the system is setup and there is at least one test that uses it and passes.

Should likely be done after #861