Tests: Improve Reliability

[n7studios]

Summary

Improves reliability of end to end tests in WordPress 6.9 RC3, by:

• improving Plugin deactivation routine to avoid timeouts and Codeception stale reference errors,
• using improved chromedriver flags to run in headless mode

This PR backports test changes to 6.8.3, to ensure they still work.

Testing

Existing tests pass.

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)