Forec installation of PHPUnit.

Author: Maxime Rainville

composer.json

@@ -43,7 +43,8 @@
         "silverstripe/dynamodb": "5.x-dev",
         "silverstripe/linkfield": "4.x-dev",
         "silverstripe/framework": "dev-pulls/5/alt-attribute-html as 5.x-dev",
-        "silverstripe/assets": "dev-pulls/2/render-image-unify as 2.x-dev"
+        "silverstripe/assets": "dev-pulls/2/render-image-unify as 2.x-dev",
+        "phpunit/phpunit": "^9.6"
     },
     "require-dev": {
         "silverstripe/frameworktest": "^1",