test: test `handle_finality_signature`

[SebastianElvis]

Existing tests for finality signatures focus on the DB access, but do not test handle_finality_signature as a whole. We still need a unit test for the entire handle_finality_signature function and its expected behaviour.

One example: #51 (comment)

[gusin13]

This ticket is much more work than I thought, to test the handle_finality_signature in rust tests properly

-we need to properly implement mocking (to mock the babylon grpc queries, like query_finality_provider, get_last_finalized_epoch)
-then insert random data in contract storage
-then finally calling contract exec msg SubmitFinalitySignature

I am thinking to leave this for now, we already have testing for inner functions and the go e2e tests

The inner function tests are already implemented in code 🤔