It was possible for the "local_socket_read" and "local_socket_write"
tests to fail because we had exited the EventLoop before
BackgroundAction got around to invoking the completion callback.
The crash happened when trying to deferred_invoke() on the background
thread, calling Core::EventLoop::current() after said EventLoop had
returned from exec().
Fix this by not passing a completion callback, since we didn't need
one in the first place.
