Fix a part of test_encode_decode_with_ecdsa_sha384 so that it actually uses the right key when testing passing a SSH public key as a string argument to decode()

Fixed a bug in the `test_encode_decode_with_ecdsa_sha384` test where the public key that was generated from using `load_pem_public_key()` at an earlier phase of the test is accidentally used instead of the string that is read from `ec_pub_file` which kind of defeats the point of that stage of the test.

Tests still pass though... so that's good! :-)