Some of the tests in PlaintextConsumer can't seem to deterministically invoke and verify the consumer callback

I was running the PlaintextConsumer to test the async consumer; however, a few tests were failing with not being able to verify the listener is invoked correctly

For example `testPerPartitionLeadMetricsCleanUpWithSubscribe`

Around 50% of the time, the listener's callsToAssigned was never incremented correctly.  Event changing it to awaitUntilTrue it was still the same case

consumer.subscribe(List(topic, topic2).asJava, listener)
val records = awaitNonEmptyRecords(consumer, tp)
assertEquals(1, listener.callsToAssigned, "should be assigned once")