test_update_image is used to test image update API, it's quite
confusing that the upload/download image API is called.
And there is the test for upload/download image API already:
test_register_upload_get_image_file
We should move the upload/download action to the right place.
Change-Id: I18e13cdb81c5ec3fc4abcdd8c8fb21eaf05d4675