ssh: Drop duplicated SSH keys from unit test
ssh: Drop duplicated SSH keys from unit test

These strings are copies of the files in
src/ssh/{test_rsa,mock_dsa_key}, so read the files instead.

Convert the two tests to automatic memory management.

martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit