tests: Log information about ssh, add comments to test.

* tests/openpgp/ssh-import.scm (ssh-version-string): New variable, and
log the binary and version used in the test.
(ssh-supports?): Document how we test what algorithms are supported by
ssh, and log ssh-keygen's replies.

We have some trouble with this test on macOS, and adding some more
information in verbose mode will hopefully make tracking down these
problems easier in the future.

• GnuPG-bug-id: T2980
• Signed-off-by: Justus Winter <justus@g10code.com>