Add test for first signer set

[friedger]

Description

This PR adds a test for clarity to calculate the current signer from the first signer set at launch of sbtc

Changes

• added new file

Testing Information

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[djordon]

This largely looks good. The only worry is that this test gives the wrong impression about the testnet address for the sbtc signers. I would add a comment that clarifies that the testnet address here is not the one to actually expect on testnet.

[djordon]

I suppose having it here is fine, but this isn't the sbtc signers' address on testnet. I would update the comment above to say that this is what the signers' address would be on testnet if the above public keys were used there. The mainnet one is the correct one though.