How to use backdoor in integration tests with 8.x.x?

I am trying to set up integration tests with v. 8.1.11. PageObjects works fine, but Backdoor doesn’t. Every backdoor operation throws a 404. Here is the test:

package it.hk.com.epictechnology.plugins.rest;

import com.atlassian.jira.pageobjects.JiraTestedProduct;
import com.atlassian.jira.pageobjects.pages.admin.ViewProjectsPage;
import com.atlassian.pageobjects.TestedProductFactory;
import org.junit.Before;
import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class RerankRequestControllerFuncTest {

    public JiraTestedProduct jira = TestedProductFactory.create(JiraTestedProduct.class);

    @Before
    public void setup () {
        jira.backdoor().subtask().enable();
    }

    @Test
    public void projectCountIsValid() {
        ViewProjectsPage viewProjects = jira.gotoLoginPage().loginAsSysAdmin(ViewProjectsPage.class);
        assertEquals(1, viewProjects.getProjects().size());
    }
}

This is the error:

  <testcase name="projectCountIsValid" classname="it.hk.com.epictechnology.plugins.rest.RerankRequestControllerFuncTest" time="2.358">
    <error message="Client response status: 404" type="com.sun.jersey.api.client.UniformInterfaceException">com.sun.jersey.api.client.UniformInterfaceException: Client response status: 404
	at it.hk.com.epictechnology.plugins.rest.RerankRequestControllerFuncTest.setup(RerankRequestControllerFuncTest.java:24)
</error>
    <system-out><![CDATA[	.	 Backdoor POST   in  1086ms  /subtask
]]></system-out>
  </testcase>