log_message_collector: assert that the log file exists if it has only started

This will fail, because that is currently not the case. However, the
following commit will make the tests pass.

ofborg/src/tasks/log_message_collector.rs

@@ -398,6 +398,7 @@ mod tests {
                 })
             );
 
+            assert!(p.path().join("routing-key-foo/attempt-id-foo").exists());
             assert_eq!(vec![worker::Action::Ack], worker.consumer(&job));
 
             logmsg.line_number = 5;