avoid packaged phpunit version that is incompatible with older PHP releases

.github/workflows/tests.yml

@@ -26,7 +26,8 @@ jobs:
       with:
         php-version: ${{ matrix.php-versions }}
         extensions: gd, sqlite3
-        tools: phpunit
+    - name: Setup PHPunit
+      run: composer install -n
     - name: Run unit tests
       run: phpunit --no-coverage
       working-directory: tst