Bugfix/fix CI

[Lewiscowles1986]

I Noticed this 2018 commit is broken, and that tests cannot be run against PHP 8.1. It also does not document which PHP Unit version the tests were for.

This does a few things:

• Yoast polyfill so PHP 5.4 - 8.1 compatible PHPUnit should "just work"
• use namespace for TestCase instead of older globally available classname
• Basic gitignore for the vendor and lock file
• update composer.json to indicate PHP 5.4+ (this is very old)
• require_once instead of require (seems to avoid double-loading file)

I Can make a separate PR if required / desired to update the GitHub action