Add automated tests for use in development

[ghost]

Is your feature request related to a problem? Please describe.
It takes a bit of time to test out module functionality when adding new features or fixing bugs, especially when changes affect all platform versions. It would be handy if testing the module during development could be automated.

Describe the solution you'd like
Functional tests that verify the module works when changes are added. The main steps that could be automated are as follows: install module, verify credentials, subscribe using form, run (and validate) customer sync, run (and validate) abandoned cart.

Describe solutions you've considered
I've looked into Codeception, it looks promising as a platform for running function/acceptance tests.

Additional context
I've had issues setting Codeception up with docker, only got it working on a local dev environment. This needs to be further researched.