I am getting the same exception (junit tests) and I am sure that it is because of the setup in persistence configuration.
Before I used the LocalTaskService I downloaded and managed to deploy the Human Task Service war. The points to note in its persistence.xml are:
<persistence-unit name="org.jbpm.task" transaction-type="RESOURCE_LOCAL">
<non-jta-data-source>java:/jdbc/jbpm-ds</non-jta-data-source>
...
<property name="hibernate.transaction.manager_lookup_class" value="org.hibernate.transaction.JBossTransactionManagerLookup" /> <!-- Jboss 5-->
I recently moved to jbpm 5.2 and have started using the LocalTaskService and need to setup the task persistence-unit correctly (and so I can use a test version in vanilla junit tests too).