tests/key.scm: Don't test ECDSA keys if GCrypt is used

* tests/key.scm: Don't do ECDSA key tests if libssh was compiled with
  GCrypt.
  (when-openssl): New macro.
  ("private-key-from-file", "public-key-from-file", "key?")
  ("private-key->public-key", "get-key-type", "public-key->string")
  ("string->public-key"): Use it.