Fix tests

[n7studios]

Summary

• Updates unit tests to be compatible with PHPUnit 9.x
• Improves test coverage
• Move environment configuration into .env files, instead of a hard coded PHP file
• Defines the PSR-12 coding standard for tests
• Adds a TESTING.MD file to advise how to write tests
• Adds the pull request template, so future PR's are pre-populated with this style of content (summary, testing, checklist etc)
• Adds a GitHub action to automatically run Coding Standard and Unit tests when a PR is created or merged

Testing

• testGetResource fails, due to code using a third party library that's reliant on the (not recommended) file_get_contents(). A separate PR to fix the code will result in this test passing.
• Tests for fetching a subscriber by email are removed for now, due to the inefficient get_subscriber_id() method attempting to fetch every single subscriber before finding a match by email (reported here, separate PR to follow to fix).

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)