CI: split unit testing in different jobs

## Task summary

1. Split unit testing in different jobs in the `unit-tests` workflow, by running one job per top-level package (i.e. one job for `lib/grandpa`)
2. Merge code coverage with https://github.com/wadey/gocovmerge before uploading it to codecov

**Why**:

- Unit testing takes 25 minutes overall, so parallelising it will make it quicker, even with the new job overhead
- Unit testing sometimes hangs forever, and we don't know why. It would be useful to be able to isolate where the fault is coming from and from which package

If one day we manage to have stable and fast (<2 minutes) unit testing, we can revert this and have it back to a single `go test ./...`.