We want to be able to use @SpringBootTest tests that fully initialize
the Spring application. This is much easier done with Junit than TestNG.
Gradle does not support (at least not easily) to run Junit and TestNG
tests. Therefore we switch to Junit with all tests.
The original reason for using TestNG was that Junit didn't support
data providers. But that finally changed in Junit5 with
ParameterizedTest.
