Add multisite testing to GitHub Actions

An addition to the test matrix in the GitHub Actions configuration for PHPUnit was made to include multisite setups. This change ensures that the codebase is compatible with WordPress multisite environments.

.github/workflows/phpunit.yml

@@ -10,6 +10,7 @@ jobs:
   php-tests:
     strategy:
       matrix:
+        multisite: [0, 1]
         php: ['8.2', '8.1', '8.0', '7.4']
         wordpress: ["latest"]
     if: github.event.pull_request.draft == false