[Event Hubs Client] Fix Flaky Procesor Test (Azure#21441)

The focus of these changes is to fix a flaky test resulting from a
recent change to the sequence of events when an event processor is closed.

Author: jsquire

sdk/eventhub/Azure.Messaging.EventHubs/tests/Primitives/EventProcessorTests.PartitionProcessing.cs

@@ -232,17 +232,22 @@ public async Task CreatePartitionProcessorDoesNotAllowReadingLastEventProperties
 
             var completionSource = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
             var options = new EventProcessorOptions { TrackLastEnqueuedEventProperties = true };
+            var mockConsumer = new Mock<TransportConsumer>();
             var mockConnection = Mock.Of<EventHubConnection>();
             var mockProcessor = new Mock<EventProcessor<EventProcessorPartition>>(5, "consumerGroup", "namespace", "eventHub", Mock.Of<TokenCredential>(), options) { CallBase = true };
 
+            mockConsumer
+                .SetupGet(consumer => consumer.IsClosed)
+                .Returns(true);
+
             mockProcessor
                 .Setup(processor => processor.CreateConnection())
                 .Returns(mockConnection);
 
             mockProcessor
                 .Setup(processor => processor.CreateConsumer(It.IsAny<string>(), It.IsAny<string>(), It.IsAny<EventPosition>(), mockConnection, It.IsAny<EventProcessorOptions>()))
-                .Returns(default(TransportConsumer))
-                .Callback(() => completionSource.TrySetResult(true));
+                .Callback(() => completionSource.TrySetResult(true))
+                .Returns(mockConsumer.Object);
 
             var partitionProcessor = mockProcessor.Object.CreatePartitionProcessor(new EventProcessorPartition(), cancellationSource, EventPosition.Earliest);