Add tests for Lab model

What does this mr do?

This MR adds tests for the Lab model

CHANGELOG.md

@@ -29,6 +29,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Show date on pending time slot request [@tnulle](https://gitlab.ewi.tudelft.nl/tnulle)
 - Add customisation for email suffix, institute name and logo [@tnulle](https://gitlab.ewi.tudelft.nl/tnulle)
 - Add statistics about assistants per lab under course status. [@tnulle](https://gitlab.ewi.tudelft.nl/tnulle)
+- Add warning messages whenever a course with groups is created, but no groups exist [@tnulle](https://gitlab.ewi.tudelft.nl/tnulle)
 
 ### Changed
 - Unauthorized students for labs/courses now redirects to enroll pages by [@cedricwilleken](https://gitlab.ewi.tudelft.nl/cedricwilleken)