Did you set the Log level REST consume to Trace so you can monitor the responses? From your console you can then inspect the data. If data was send you might find here the reason why the mapping failed. But my gut feeling is that no data was send.
Your post is saying you want to retrieve some data but your REST action is PUT instead of GET.
All seems to be well. Your response is empty because it returns the Root-entity. The data you are looking for has been stored to the database by the import mapping.
What you need to do in your microflow is retrieve the TestEntity over association of the Root.