Update integration test to prevent regression

[TWiStErRob]

• Tests #498

• [x] Can you back your code up with tests?

• [x] Keep your commits clean: squash your commits if necessary.

I created a separate PR as this might be contested and iterated, and don't want to block the merging of the fix.

[TWiStErRob]

This might be superseded by https://github.com/mockito/mockito-kotlin/pull/502, which a much more robust and realistic way of testing a library.

[TimvdLippe]

Let's do #502 instead indeed!