test(signer): Add test with duplicate signer private key

[jbencin]

Description

• Add v0 signer test where first two signers have the same key
• Add checks in mine_and_verify_confirmed_naka_block() to confirm that no two signers have the same private key

Applicable issues

• fixes #5077

Additional info (benefits, drawbacks, caveats)

I was trying to make this test check that only the first signer with the duplicate was accepted, and the second was rejected, but this seems impossible because the signatures from the signers are identical. So I just check that the signature is not included twice

Checklist

• [x] Test coverage for new or modified code paths
• [x] New integration test(s) added to bitcoin-tests.yml