This one sometimes fails with a zombie detected, so I'm assuming it's
too fast for reaping to happen, so we add another 100ms sleep.
Yeah, this isn't great but...eh
This allows code of the form `if jobs -q $some_pid` in scripts to check whether a previously started job is still running. Previously this would return the correct value, but also print an error message.
The invalid argument errors will still be printed.
Added test cases for both.
