test-crypto-secret: add 'secret_keyring' object tests.

test-crypto-secret: add 'secret_keyring' object tests.

Add tests:
  test_secret_keyring_good;
  test_secret_keyring_revoked_key;
  test_secret_keyring_expired_key;
  test_secret_keyring_bad_serial_key;
  test_secret_keyring_bad_key_access_right;

Added tests require libkeyutils. The absence of this library is not
critical, because these tests will be skipped in this case.

Signed-off-by: Alexey Krasikov <alex-krasikov@yandex-team.ru>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>