Unit test batch signed data set logic

[corverroos]

🎯 Problem to be solved

We implemented batched signed data set handling in parsigdb, sigagg, aggsigdb, recaster and bcast in #2373. But we didn't unit test the actual handling of batches, we only test legacy single use-cases.

🛠️ Proposed solution

Write unit tests to checks whether batch logic is correct and does proper error handling in the event that a single instance in the batch errors.