ssh signing: verify ssh-keygen in test prereq

Do a full ssh signing, find-principals and verify operation in the test
prereq's to make sure ssh-keygen works as expected. Only generating the
keys and verifying its presence is not sufficient in some situations.
One example was ssh-keygen creating unusable ssh keys in cygwin because
of unsafe default permissions for the key files. The other a broken
openssh 8.7 that segfaulted on any find-principals operation. This
extended prereq check avoids future test breakages in case ssh-keygen or
any environment behaviour changes.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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