This commit migrates the test to a brand new V2, F38-based manifest. It's
actually based on osbuild-composer interpretation of the Fedora Cloud Base
image.
Require all the tests that compile a manifest to either specify
checkpoints or exports. Convert all the tests that were relying
on implicit exports with v1 manifests to use explicit exports.
Remove the dependency on unittest for the `OSBuild` class which
used the `unittest` instance only for `assertEqual`, which can
easily also be done via a plain `assert`.
Previously, the osbuild executor had its internal temporary directory that
served as the output directory. However, this approach gives no power to
the caller to control the lifetime of the produced artifacts. When more
images are built using one executor, the results will accumulate in one
place possibly leading to exhaustion of disk space.
This commit removes the executor's internal output directory. The output
directory can now be passed to osbuild.compile, so the caller can control
its lifetime. If no directory is passed in, the compile method will use
its own temporary directory - this is useful in cases when the caller
doesn't care about the built artifacts or the manifest doesn't have any
outputs.
