Fixed Severity `Major` from Static Code Analysis

Sure, but the original variable `$queuePluginManager` is not used anywhere. Then the usage of `$this->queue` on `line 30` should be renamed to `$this->queuePluginManager` https://github.com/eddiejaoude/SlmQueue/blob/fix-serverity-major/tests/SlmQueueTest/Controller/AbstractControllerTest.php#L30