Allow tests to be run on any image change

### Expected behavior
When defining tests allow the image reference to be either empty or not provided. This would cause tests to run on every iteration. I'd like to be able to setup remote modules with shared tests steps but at the moment you can't do that because the module would have to explicitly call out an image.

### Actual behavior
Image field must be set in the test section so tests cannot be run or shared across repos.