Tests: Fix chromedriver crashes

[n7studios]

Summary

Fixes backward compat. tests from failing due to chromedriver crashing:

--disable-renderer-backgrounding: prevents Chrome from throttling or suspending renderer processes for tabs it considers "backgrounded." In headless test environments, Chrome can sometimes mistakenly treat the active tab as backgrounded, causing it to deprioritise the renderer and eventually time out.
--disable-backgrounding-occluded-windows: similar to above but specifically for windows Chrome considers "occluded" (hidden behind other windows).
--js-flags=--max-old-space-size=512: sets the V8 JavaScript engine's maximum old-generation heap size to 512MB. This forces more aggressive garbage collection rather than letting V8 grow its heap unbounded.

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)