N°2651 rollback gitignore for lib tests dirs

Too dangerous ! We'll work properly on this but for 2.8

lib/symfony/framework-bundle/Tests/Functional/app/Serializer/config.yml

@@ -0,0 +1,10 @@
+imports:
+    - { resource: ../config/default.yml }
+
+services:
+    _defaults: { public: true }
+    test.property_info: '@property_info'
+
+framework:
+    serializer: { enabled: true }
+    property_info: { enabled: true }