tests/functional: Convert the s390x avocado tests into standalone tests

tests/functional: Convert the s390x avocado tests into standalone tests

These tests use archive.lzma_uncompress() from the Avocado utils,
so provide a small helper function for this, based on the
standard lzma module from Python instead.

And while we're at it, replace the MD5 hashes in the topology test
with proper SHA256 hashes, since MD5 should not be used anymore
nowadays.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240830133841.142644-21-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>