@Override
  public <StreamMessageType> InMemoryOutputDescriptor<StreamMessageType> getOutputDescriptor(
      String streamId, Serde<StreamMessageType> serde) {
    return new InMemoryOutputDescriptor<StreamMessageType>(streamId, this);
  }
Constructs a new InMemorySystemDescriptor from specified components.
<p>
Every {@link InMemorySystemDescriptor} is configured to consume from the oldest offset, since stream is in memory and
is used for testing purpose. System uses {@link InMemorySystemFactory} to initialize in memory streams.
<p>
@param systemName unique name of the system
samza-test/src/main/java/org/apache/samza/test/framework/system/descriptors/InMemorySystemDescriptor.java