Enforce PHPUnit 8 for PHP >= 8.1

Author: kylekatarnls

.github/workflows/tests.yml

@@ -38,7 +38,9 @@ jobs:
 
     - name: Install dependencies
       if: steps.composer-cache.outputs.cache-hit != 'true'
-      run: composer update --prefer-dist --prefer-${{ matrix.setup }} --no-progress --no-interaction ${{ matrix.php >= 8.1 && '--ignore-platform-req=php' || '' }}
+      run: |
+        ${{ matrix.php >= 8.1 && 'composer require --no-update phpunit/phpunit:^8.5.15 --no-interaction;' || '' }}
+        composer update --prefer-dist --prefer-${{ matrix.setup }} --no-progress --no-interaction ${{ matrix.php >= 8.1 && '--ignore-platform-req=php' || '' }}
 
     - name: Run test suite
       run: vendor/bin/phpunit --no-coverage --verbose