Improve plugin tests

[capuderg]

Currently, there are a few PHPunit tests written for this plugin, but the setup for running this tests is a bit outdated and it only supports unit tests.

It would be great to migrate to WP Browser (codeception). This would allow for easier local setup and running the test and we could also write acceptance tests to cover the plugin UI and functionality.

• configure WP-browser (codeception) for running wp unit tests and acceptance tests with
• migrate existing PHPunit tests to run on WP-browser (codeception)
• write acceptance tests to cover as much functionality as possible