taskService.setVariable(task.getId(), "testVar", "testValue");
HistoricVariableInstanceQuery query = historyService.createHistoricVariableInstanceQuery();
assertEquals(1, query.count());
HistoricVariableInstance variable = query.singleResult();
// the variable is in the process instance scope
assertEquals(pi.getId(), variable.getActivityInstanceId());
taskService.complete(task.getId());
assertProcessEnded(pi.getId());