Update tests to check the image behavior

This splits the current test image script in 2:

• one to (locally) build and test the image
• one to only test the image

This introduces a Makefile to trigger simple test targets intended to be used by the ci.

As an implementation (and gory) detail, this opens a conditional within the python
script to not do anything during the download phase when the base url starts with
test://. While i'm not a fan, it's simple enough to allow this make test to work
without too much complication.

Related to T3746