Tests: Improve nginx configuration

[n7studios]

Summary

Improves the nginx configuration in tests run by GitHub actions by:

• increasing the buffer size in nginx,
• includes uploading nginx's error.log to the GitHub action artifacts should tests in the future fail

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)