Improve test coverage for process management

[RealOrangeOne]

Ensuring the db_worker command behaves correctly is critical.

• Does it terminate cleanly on ctrl+c?
• Does it terminate cleanly on SIGTERM, SIGINT and SIGKILL?
• What if a task raises an exception it's not expected to?

All of the above should be the case whether there's a task currently running or not. And what happens to the task when this does happen?

[JaeHyuckSa]

@RealOrangeOne I saw the updates on this issue, and it looks like the current PR has covered a lot. I'd like to help—are there any remaining test cases or areas that still need attention?

[RealOrangeOne]

Windows improvements are the main things left if you fancy giving them a look: #109