[FLINK-3265][tests] adapt RMQSource checkpointing test to runtime behavior

The methods snapshotState and notifyCheckpointComplete are always
mutually exclusive. The RMQSource relies on this but the test makes a
false assumption when it calls those two methods at the same time.

This closes #1569.

flink-streaming-connectors/flink-connector-rabbitmq/src/test/java/org/apache/flink/streaming/connectors/rabbitmq/RMQSourceTest.java

@@ -140,7 +140,9 @@ public class RMQSourceTest {
 			}
 
 			// check if the messages are being acknowledged and the transaction comitted
-			source.notifyCheckpointComplete(snapshotId);
+			synchronized (DummySourceContext.lock) {
+				source.notifyCheckpointComplete(snapshotId);
+			}
 			totalNumberOfAcks += numIds;
 
 		}