add test for OpenSSH DSA keys

OpenSSH 9.8 disabled by default support for DSA keys, including handling in ssh-keygen that we were using. They also plan to completely drop the support sometime in 2025.

For now we have disabled the test when detecting ssh >> 9.7p1, but we should find a way to process these files even with some different tool, and/or provide a better error handling at the comparator level.

See #382 (closed) and !139 (closed) and 0743d63f and f3f72b9f and 652b5793 and 7917b746