[main] DEBUG JobExecutor : starting thread group 'null'...
The message is not important. 'null' is just the name of JobExecutor.
First, I think you don't have to configure your own jbpm.cfg.xml.
jbpm-jpdl.jar:org/jbpm/default.jbpm.cfg.xml has properly configured JobExecutor and will be looked up if you don't have your own jbpm.cfg.xml.
Second, your problem is that your java process ends before JobExecutor captures timer jobs. Try Thread.sleep(10000) at the end of your main method.
This ad-hoc impl is ok if you just want to see how Timer works. But in real use case, you need to make sure the JobExecutorThread is alive. That's why the jbpm distribution contains job executor launcher servlet which run on an application server.