Acceptance tests: prevent deadlocks in tests for async writes

[hariso]

Feature description

In two of our tests for async writes, we are waiting for all the ack functions to be called. To prevent deadlocks, we need to specify a timeout, so that tests do not wait indefinitely, in cases where there's actually a bug in the connector code.

[hariso]

I actually faced this when working on async writes for the Kafka connector. I didn't call the ack functions, the acceptance test was stuck, and it wasn't immediately clear why. The fix is a quick one.

[neovintage]

This doesn't happen often enough to be material. Will look into this again if it becomes more of a problem.