Add tests for Lab model

What does this mr do?

This MR adds tests for the Lab model

CHANGELOG.md

@@ -58,6 +58,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Get Next button will only show up if the lab is open. [@tnulle](https://gitlab.ewi.tudelft.nl/tnulle)
 - Improve performance of counting the number of requests current in the queue for a specific assistant and specific lab by [@lemaire](https://gitlab.ewi.tudelft.nl/lemaire)
 - Add toString method to FirstYearStudent to fix StackOverFlowError when calling firstYearStudent.toString() [@tnulle](https://gitlab.ewi.tudelft.nl/tnulle)
+- Get Next button will only show up if the lab is open. [@tnulle](https://gitlab.ewi.tudelft.nl/tnulle)
 
 ## [1.1.0] - (2019-12-16)
 ### Deprecated