PlaintextConsumerTest needs to wait for reconciliation to complete before proceeding

Several tests in PlaintextConsumerTest.scala (such as testPerPartitionLagMetricsCleanUpWithSubscribe) uses:

assertEquals(1, listener.callsToAssigned, "should be assigned once")

However, as the timing for reconciliation completion is not deterministic due to asynchronous processing. We actually need to wait until the condition to happen.

However, another issue is the timeout - some of these tasks might not complete within the 600ms timeout, so the tests are deemed to be flaky.