Try with
EntityManagerFactory emf = Persistence
.createEntityManagerFactory("org.jbpm.persistence.jpa");
"org.jbpm.task" should be used to construct your task service, not to create the session (for example, https://github.com/calcacuervo/JBPM5-Samples/blob/master/human-tasks/src/test/java/com/test/BaseHumanTaskTest.java)
Demian