When I had this error it was because my test images weren't in a subfolder within the test folder. My problem was solved once I moved the images from ./test to ./test/unknown.
The dogs_cats_redux notebook has an example of using the Vgg16.test method, in case you have other issues. (Disclaimer: The notebook might contain spoilers that could rob you of useful experiences.)
The way I found out what to do was to search "test" within the Lesson 1 discussion, but it took some digging to find a reply that answered my question.
Here's the reply that answered my question: