Add test for keymgr interop with external tools like ssh-keygen

tor-keymgr should have some ssh-keygen inerop tests. Currently, the keymgr tests use keys generated using maint/keygen-openssh-test/generate. Ideally we should have some tests that check the keymgr is able to handle keys generated by external tools (mainly ssh-keygen) as well.

This ticket will likely involve:

• moving the keys from testdata into testdata/arti (and updating the paths from maint/keygen-openssh-test/generate accordingly)
• extending maint/keygen-openssh-test/generate to generate a set of test keys using ssh-keygen into testdata/ssh-keygen)
• updating the keymgr tests to run against both sets of keys, not just testdata/arti (tesdata/arti and testdata/ssh-keygen)

Bonus: we should have a CI job that ensures running maint/keygen-openssh-test/generate doesn't change tesdata (i.e. that what's checked into the repo is exactly what the script would generate).

See also this thread from !2121 (merged):

• @Diziet started a discussion: (+4 comments)

  I think one downside of this is that we no longer test actual interoperability between our tools and ssh-keygen? I don't think that's a blocker for this MR but it would probably be good to add some tests that did that.

  I guess such tests would have to generate keys at runtime and check they appear to work after round trips.

  If you agree with me on this point, how about filing a ticket for that?