Don't abuse the API when that's not what is tested

Don't abuse the API when that's not what is tested

test_EVP_PKEY_CTX_get_set_params() in test/evp_extra_test.c abused
previously sloppy checking in EVP_PKEY_sign_init_ex(), by passing a
"key to sign with" that was really just domain parameters.

Now that underlying provider import of key payload has become a bit
more strict, that leads to errors, so we need to provide at least a
public part (even though fake), and because this is a signing
operation, a private part as well.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10169)