Add Abandoned Cart Functionality Tests

[n7studios]

Summary

Adds tests for #259

Changes the hooks used on Plugin activation/deactivation to register the action scheduler's action, to ensure this works.

Testing

• AbandonedCartCest: Test that the abandoned cart functionality works
• UpgradePathsCest: Test that the abandoned cart action is scheduled on upgrade

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)