You will need the MockProcess from https://github.com/camunda/camunda-bpm-mockito/blob/master/src/test/resources/MockProcess.bpmn

CandidateAssigneDoesNotWorkTest.java

package org.camunda.bpm.extension.mockito;

import org.camunda.bpm.engine.delegate.DelegateTask;
import org.camunda.bpm.engine.delegate.TaskListener;
import org.camunda.bpm.engine.runtime.ProcessInstance;
import org.camunda.bpm.engine.test.Deployment;
import org.camunda.bpm.engine.test.ProcessEngineRule;
import org.junit.Before;
import org.junit.Rule;
import org.junit.Test;

import static org.camunda.bpm.engine.test.assertions.ProcessEngineTests.assertThat;
import static org.camunda.bpm.engine.test.assertions.ProcessEngineTests.claim;
import static org.camunda.bpm.engine.test.assertions.ProcessEngineTests.complete;
import static org.camunda.bpm.engine.test.assertions.ProcessEngineTests.runtimeService;
import static org.camunda.bpm.engine.test.assertions.ProcessEngineTests.task;

/**
 * Created by jangalinski on 04.11.15.
 */
public class CandidateAssigneDoesNotWorkTest {

  @Rule
  public final ProcessEngineRule processEngineRule = new ProcessEngineRule(new MostUsefulProcessEngineConfiguration().buildProcessEngine());

  @Before
  public void setUp() {
    DelegateExpressions.registerExecutionListenerMock("startProcess");
    DelegateExpressions.registerExecutionListenerMock("beforeLoadData");
    DelegateExpressions.registerJavaDelegateMock("loadData");
    DelegateExpressions.registerJavaDelegateMock("saveData");

  }


  @Test
  @Deployment(resources = "MockProcess.bpmn")
  public void does_not_work() {
    Expressions.registerInstance("verifyData", new TaskListener() {
      public void notify(DelegateTask delegateTask) {

        delegateTask.addCandidateGroup("group");
        delegateTask.setAssignee("user");
      }
    });


    final ProcessInstance processInstance = runtimeService().startProcessInstanceByKey("process_mock_dummy");

    assertThat(processInstance).isWaitingAt("UserTask_1");

    assertThat(task()).hasCandidateGroup("group");
    assertThat(task()).isAssignedTo("user");

    complete(task());
    assertThat(processInstance).isEnded();
  }

  @Test
  @Deployment(resources = "MockProcess.bpmn")
  public void does_work() {
    Expressions.registerInstance("verifyData", new TaskListener() {
      public void notify(DelegateTask delegateTask) {

        delegateTask.addCandidateGroup("group");
      }
    });


    final ProcessInstance processInstance = runtimeService().startProcessInstanceByKey("process_mock_dummy");

    assertThat(processInstance).isWaitingAt("UserTask_1");

    claim(task(), "user");

    assertThat(task()).hasCandidateGroup("group");
    assertThat(task()).isAssignedTo("user");

    complete(task());
    assertThat(processInstance).isEnded();
  }


}