[Chore]: Fix flaky test transaction_coordinator::sign_bitcoin_transaction

[MCJOHN974]

Bug - Fix flaky test transaction_coordinator::sign_bitcoin_transaction

1. Description

I found that this test is actually flaky, workflow where it fails:

https://github.com/stacks-sbtc/sbtc/actions/runs/16723011611/job/47331584358

2. Why It's Necessary

We usually do not tolerate flaky tests in sBTC.

3. Execution Checklist

• debug & research why it happens
• fix the test

[djordon]

It failed on me too 😞 https://github.com/stacks-sbtc/sbtc/actions/runs/16730112099/job/47355872487