The stage testing is based on an output from the tree-diff tool. During
one test two pipelines are run and their outputs are compared using
tree-diff. The diff is then compared with expected diff included in
the repository.
Use the unittest module from the standard library. Also, ensure that
separate runs of this test don't share a osbuild store and clean up
after themselves.
With contributions from Ondřej Budai and Tom Gundersen.
