ci: Enforce the CI to test against Symfony5 (#1817)

Depends on #1815.

Adapt the CI to enforce testing against Symfony5 and allowing to easily add a build against Symfony6, preparing #1775.

.github/workflows/tests.yaml

@@ -21,13 +21,16 @@ jobs:
         # TODO: include highest & lowest
         dependencies: [ locked ]
         coverage-driver: [ pcov, xdebug ]
+        # TODO: include Symfony6 later
+        symfony-version: [ '5.4.*' ]
         include:
           - operating-system: windows-latest
             php-version: '8.0'
             dependencies: locked
             coverage-driver: xdebug
+            symfony-version: '5.4.*'
 
-    name: CI on ${{ matrix.operating-system }} with PHP ${{ matrix.php-version }} (${{ matrix.dependencies }}), using ${{ matrix.coverage-driver }}
+    name: Tests on ${{ matrix.operating-system }} with PHP ${{ matrix.php-version }} (${{ matrix.dependencies }}; Symfony ${{ matrix.symfony-version }}), using ${{ matrix.coverage-driver }}
 
     steps:
       - name: Checkout code
@@ -45,6 +48,17 @@ jobs:
         if: matrix.dependencies != 'locked'
         run: composer config --unset platform.php
 
+      - name: Enforce the Symfony version used
+        if: ${{ matrix.symfony-version }}
+        run: composer config extra.symfony.require ${{ matrix.symfony-version }}
+
+      # See https://symfony.com/doc/current/bundles/best_practices.html#require-a-specific-symfony-version
+      - name: Install Flex
+        if: ${{ matrix.symfony-version }}
+        run: |
+            composer global config --no-plugins allow-plugins.symfony/flex true
+            composer global require --no-progress --no-scripts --no-plugins symfony/flex
+
       - name: Get composer cache directory
         id: composer-cache
         shell: bash