[BEAM-3598] Sickbay Kinesis ShardReadersPoolTest

[tgroh]

shouldStopReadersPoolAlsoWhenExceptionsOccurDuringStopping is quite
flaky, due to missing mock interactions.

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[kennknowles]

Right back at you: #4605

[tgroh]

Yeh, got crushed by LIFO review inspections.

GG no re, and whatnot.