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?