Remove phpstan errors from the rest of the tests

[MAILPOET-3235]

tests/integration/Settings/UserFlagsControllerTest.php

@@ -71,6 +71,7 @@ class UserFlagsControllerTest extends \MailPoetTest {
       'userId' => $this->currentUserId,
       'name' => 'flag_1',
     ]);
+    assert($flag instanceof UserFlagEntity);
     expect($flag->getValue())->equals('updated_value');
   }