Increase test coverage

Add some more tests to increase test coverage. Taken from this table: https://scrutinizer-ci.com/g/juriansluiman/SlmQueue/code-structure/master?elementType=class&orderField=test_coverage&order=asc&changesExpanded=0

 - [x] JobPluginManager
 - [x] AbstractJob
 - [x] WorkerEvent
 - [x] QueuePlugin
 - [x] AbstractWorker