Integration tests have sync issues producing meaningless tracebacks and error messages

[ptormene]

It seems related to the concurrency between loaders, while a loader is launched and the previous one has not been completed yet.
We probably need to improve signals in order to keep track of which loader was started, so we can know which loader has completed or failed its execution.

If we run integration tests like this:
ONLY_OUTPUT_TYPE=avg_losses-rlzs ONLY_CALC_ID=29 ./run_integration_tests.sh
and we make sure only one taxonomy is tested, the right traceback is shown.