Fix occasionaly failing TestBroadcast* tests
Fix occasionaly failing TestBroadcast* tests

This changeset removes a race condition responsible for the ocassional
failures we see in CI in the TestBroadcast* tests that read from "disk",
the construct that represents the broker's disk where all produced
messages are eventually written to.

The TestBroadcast* tests have been executed 10000 times and do not fail.

Change-Id: I5dea63a52d64171bfd703e5cd053437b0d215cf8
Signed-off-by: Kostas Christidis <e1a8399e36effe75a124102eb2a2bdfc82e0aab4@christidis.io>

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