Tests must be invoked with a qcow2 test subject

[stefwalter]

The tests must be invoked with a test subject, and not a running Atomic Host.

Each change (package or otherwise) composed in the pipeline should prepare a test subject (ie: qcow):

https://fedoraproject.org/wiki/Changes/InvokingTests

The tests are invoked with that test subject, which will then launch and/or consume it as appropriate. Multiple techniques are available to do this, among which might be:

• Recomposing a qcow2 each time (expensive)
• Running an rpm-ostree rebase inside an existing qcow2
• Running an rpm-ostree install inside an existing qcow2

[stefwalter]

Some partial work on this is done here: #383, but as @johnbieren pointed out when looking at that pull request. More work is necessary.

[arilivigni]

This is completed:

#395