Tests downstream without ssh keygen

[blackboxsw]

Proposed Commit Message

    tests: skip azure ssh-keygen unittests when ssh-keygen not installed
    
    Followup to baedc33ac to support skipping azure ssh-keygen related
    tests from running in environments without ssh-keygen installed.

Additional Context

Daily recipe builds fail for resolute because the default images created do not have ssh-keygen installed
https://launchpadlibrarian.net/836531696/buildlog_ubuntu-resolute-amd64.cloud-init_100.0.0.daily-202512092133-78a748808~ubuntu26.04.1_BUILDING.txt.gz

                if err_filename is not None:
>                   raise child_exception_type(errno_num, err_msg, err_filename)
E                   FileNotFoundError: [Errno 2] No such file or directory: b'ssh-keygen'

/usr/lib/python3.13/subprocess.py:1972: FileNotFoundError

Test Steps

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

Approved once CI passes. Thanks - and agreed. Even if we end up adding this as a test dependency it's still better to keep unit tests passing independent of system binaries.