Change non-static class-level variables and methods to static in tests's data providers

The update modifies several test classes within the "chill-project" to change non-static class-level variables and methods to static ones. This change has been made to improve readability, performance, and to eliminate unnecessary instantiation of class objects in test scenarios. Also, flush and clear actions on the entity manager are moved to individual data providers.

src/Bundle/ChillMainBundle/Test/PrepareCenterTrait.php

@@ -28,7 +28,7 @@ trait PrepareCenterTrait
     /**
      * prepare a mocked center, with and id and name given.
      */
-    protected function prepareCenter(int $id, string $name): Center
+    protected static function prepareCenter(int $id, string $name): Center
     {
         $center = new Center();
         $center->setName($name);