A question about "Test Dataset"

Thanks for the amazing job. 
When I have been learning the project, I find the "Test Dataset" is 5k images of the "ILSVRC2012_img_val" Dataset in " README.md".  As we know, the "ILSVRC2012_img_val" Dataset has 50k images. 
Would you please teach me how to select 5k images in  50k images of the "ILSVRC2012_img_val"? 
Looking forward to your reply.