Add golden test for governance drep registration-certificate

[smelc]

Using @CarlosLopezDeLara's input, we can add a golden test for the relatively new drep registration-certficate command.

This issue keeps track of doing so.

➜  testdreps cardano-cli conway governance drep registration-certificate \
--drep-verification-key-file drep.vkey \
--key-reg-deposit-amt 0 \
--drep-metadata-url drep.url \
--drep-metadata-hash 52e69500a92d80f2126c836a4903dc582006709f004cf7a28ed648f732dff8d2 \
--out-file drep-reg.cert
➜  testdreps cat drep-reg.cert
{
    "type": "CertificateShelley",
    "description": "DRep Key Registration Certificate",
    "cborHex": "84108200581c95591451fb3899fcc8db3450741d9de6123a78e03ef39bf4a65ed5cf008268647265702e75726c582052e69500a92d80f2126c836a4903dc582006709f004cf7a28ed648f732dff8d2"
}

[16, [0, h'95591451FB3899FCC8DB3450741D9DE6123A78E03EF39BF4A65ED5CF'], 0, ["drep.url", h'52E69500A92D80F2126C836A4903DC582006709F004CF7A28ED648F732DFF8D2']]

Originally posted by @CarlosLopezDeLara in #330 (review)