added phpunit v9 tool

Author: connorhu

.github/workflows/continuous-integration.yml

@@ -40,6 +40,7 @@ jobs:
           php-version: "${{ matrix.php-version }}"
           extensions: apcu
           ini-values: apc.enable_cli=1
+          tools: phpunit:9
 
       - name: Get composer cache directory
         id: composer-cache
@@ -62,4 +63,4 @@ jobs:
         run: php data/bin/symfony symfony:test --trace
 
       - name: Run PHPUnit Tests
-        run: php vendor/bin/phpunit
+        run: phpunit