Add E2E tests

[iamdharmesh]

Description of the Change

PR adds some new end-to-end tests for the plugin based on the critical flow wiki.

How to test the Change

Make sure tests running properly and passing in local or in GH action.

Changelog Entry

Added - Initial E2E tests and coverage of the new OAuth connection flows.

Credits

Props @iamdharmesh

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[dkotter]

@iamdharmesh E2E tests are failing now and I think it's due to the environment variables no longer existing (like Cypress.env('MAILCHIMP_USERNAME') as an example). I don't recall how we set those previously, are those variables stored at the GitHub level that maybe got removed?

[iamdharmesh]

@dkotter, I can see the environment variables on the list. However, if you are talking about E2E tests failing in recent PRs from dependabot then it's due to repo secrets are not available for PR raised by bot or from the forked repo. So, this is expected (I am not sure on how we can make repo secrets available to the Dependabot PRs)