Add tests for all notifications

[jessedezwart]

We'll probably want to use something like Mockito.

[ikdekker]

Mockito is a mocking framework, but the obvious choice for a testing framework is JUnit 5, which we want to use in combination with Mockito. I'll look into setting this up soon.

[jessedezwart]

The spring-boot-starter-test dependency uses JUnit 5. It also includes Mockito apparently!