(PUP-2402) add sshkey spec tests

This commit adds tests for the sshkey type. They can be automated in
rspec at much lower cost than acceptance.
Convert John's earlier closed PR to spec tests. Add tests for the full
range of encryption types. Add a negative test for unknown encryption
types.
Move the test to provider directory as it's testing provider
functionality.

This commit also fixes a deprecation warning for testing file modes. It
used to use should, use expect instead.