calcs/hazard/event_based/core:

Removed the `n_sources` counter. It wasn't used in production.

It was referenced in one test, but it was used in a way that didn't make
sense.
tests/calcs/hazard/event_based/core_test:

Removed an assertion referencing the `n_sources` variable, which has been
deleted.
calcs/hazard/general:

Order results by id in `_get_realizations`.
calcs/hazard/event_based/core:

Use `_get_realizations`.
tests/calcs/hazard/general_test:

Renamed some test mock objects for better clarity.
tests/calcs/hazard/event_based/core_test:

Added a test for the `task_arg_gen` logic of the event-based calc.