[FAB-1663] Add helper functions to tests

https://jira.hyperledger.org/browse/FAB-1663

Replace often used sequences/patterns throughout the tests in the
`kafka` package with wrapper functions.

Change-Id: I5fe3c6a6221b98a17d6867aba0f40404b856bb65
Signed-off-by: Kostas Christidis <kostas@christidis.io>