Merge branch 'add/phpunit-polyfills' into 'main'

Satisfy dependencies

See merge request !13

plugins/templates/before-script.yml

@@ -13,3 +13,6 @@ before_script:
     else
       composer global require "phpunit/phpunit=4.8.*"
     fi
+
+  # PHPUnit requirements
+  - composer global require yoast/phpunit-polyfills