[ticket/14717] Quote strings beginning with @ or % in yaml definitions

This type of use was deprecated in symfony 2.8 and will be dropped in 3.x.
Also see: https://github.com/symfony/symfony/pull/16285

PHPBB3-14717

tests/di/fixtures/config/production/container/environment.yml

@@ -8,7 +8,7 @@ services:
     dbal.conn:
         class: phpbb\db\driver\factory
         arguments:
-            - @service_container
+            - '@service_container'
 
     dispatcher:
         class: phpbb\db\driver\container_mock

tests/di/fixtures/config/test/container/environment.yml

@@ -8,7 +8,7 @@ services:
     dbal.conn:
         class: phpbb\db\driver\factory
         arguments:
-            - @service_container
+            - '@service_container'
 
     dispatcher:
         class: phpbb\db\driver\container_mock

tests/di/fixtures/other_config/production/container/environment.yml

@@ -8,7 +8,7 @@ services:
     dbal.conn:
         class: phpbb\db\driver\factory
         arguments:
-            - @service_container
+            - '@service_container'
 
     dispatcher:
         class: phpbb\db\driver\container_mock

tests/di/fixtures/other_config/test/container/environment.yml

@@ -8,7 +8,7 @@ services:
     dbal.conn:
         class: phpbb\db\driver\factory
         arguments:
-            - @service_container
+            - '@service_container'
 
     dispatcher:
         class: phpbb\db\driver\container_mock