We need a JSON-RPC test for background_fetch.

I just merged #7438 but looking through the tests now I have found that we don't have any tests for it. The only way to test is manually with a REPL command "fetch" currently.

It's enough to call stop_io(), then send a message to the account, then run background_fetch() - it should receive the message. Don't know how to test stop_background_fetch(), but can also be tested that it at least allows the background_fetch() to be called afterwards and there are no deadlocks.