Make sure test case class is saved in TestingEnvironment file so proxy entry...

Make sure test case class is saved in TestingEnvironment file so proxy entry points will apply overridden DI config.

tests/PHPUnit/Framework/Fixture.php

@@ -255,6 +255,7 @@ class Fixture extends \PHPUnit_Framework_Assert
             return;
         }
 
+        $this->getTestEnvironment()->testCaseClass = $this->testCaseClass;
         $this->getTestEnvironment()->save();
         $this->getTestEnvironment()->executeSetupTestEnvHook();