[FLINK-23318][tests] Fix AkkaRpcActorTest.testOnStopFutureCompletionDirectlyTerminatesAkkaRpcActor

[tillrohrmann]

After FLINK-23202 the AkkaRcpActorTest.testOnStopFutureCompletionDirectlyTerminatesAkkaRpcActor became unstable
because it assumed that RPC call after the RpcEndpoint is terminated won't be executed. With FLINK-23202 these
calls are now sent to the dead letter box which will result in a Failure response. Consequently, the second rpc
is now failing with an RecipientUnreachableException. This commit updates the test to assert for this condition.

Before, the test was passing because of a potential race condition where the second result future was not completed.

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[rmetzger]

Test fix looks good to me!

[tillrohrmann]

Thanks for the review @rmetzger. Merging this PR now.