Add test matrix for Contao versions

.github/workflows/ci.yml

@@ -11,12 +11,13 @@ jobs:
         uses: 'terminal42/contao-build-tools/.github/workflows/build-tools.yml@main'
 
     tests:
-        name: Unit tests
+        name: Unit tests (PHP ${{ matrix.php }} / Contao ${{ matrix.contao }})
         runs-on: ubuntu-latest
         strategy:
             fail-fast: false
             matrix:
                 php: [ '8.1', '8.2', '8.3' ]
+                contao: [ '4.13', '5.3', '5.4' ]
         steps:
             - name: Setup PHP
               uses: shivammathur/setup-php@v2
@@ -27,6 +28,9 @@ jobs:
             - name: Checkout
               uses: actions/checkout@v3
 
+            - name: Require Contao version for tests
+              run: composer require contao/core-bundle:${{ matrix.contao }}.* --dev --no-update
+
             - name: Install the dependencies
               run: |
                   composer install --no-interaction --no-progress --no-plugins