Speed up kafka tests

[kubukoz]

After we added the fs2-kafka module, the tests got a little too slow - this is most likely due to starting and stopping a separate container for every test.

We should investigate ways to reduce the time it takes to run these tests: maybe running them in parallel (should be safe due to using a different container for each test), maybe ditching docker altogether and running a fake in-memory kafka instance?

FYI @mpapillon if you want to give this a try, that'd be great (but don't feel obliged :) )