flaky test: "KafkaSingletonTest"

[mr-git]

sometimes KafkaSingletonTest fails with:

[info] TEST FAILED - KafkaSingletonTest: allocate & release when partition assigned or revoked: None did not equal Some(<(), the Unit value>) (140 milliseconds)
[info]   None did not equal Some(<(), the Unit value>) (KafkaSingletonTest.scala:61)
[info]   Analysis:
[info]   None$(value: -> ())