Fix (another) intermittent test failure

We are testing, if the `send` method of the mock producer, gets called by submitting a RollupEvent to `KafkaService`. This happens in a `ThreadPoolExecutor`, so there is no guarantee, that the `send` method will be executed instantaneously.
Adding a timeout for verification, __might__ solve this problem.
If it does not, I will increase the timeout OR remove this (over) test.