Tests: Fix 502 Bad Gateway on WPForms Plugin Deactivation

[n7studios]

Summary

Tests deactivate the WPForms Lite Plugin in a different way to other Plugins, to prevent a 502 bad gateway in nginx. In another PR, configuring nginx in GitHub's actions resolved a different 502 bad gateway error, which is the inspiration for this PR.

This PR resolves by:

• increasing the buffer size in nginx,
• includes uploading nginx's error.log to the GitHub action artifacts should tests in the future fail,
• use the standard method for deactivating the WPForms Lite Plugin in tests

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)