Simplify testStep().session()

[michaeldfallen]

In test/util/supertest.js I have written a too complex function that lets users assert against the session as seen in test/steps/EntryPoint.test.js:

    it('creates a session', () => {
      return testStep(redirect)
        .get()
        .session(session => {
          expect(session.active).to.be.true;
        });
    });

It does this by issuing a get request to an endpoint that will return the session json.

If instead the DSL created it's own session store and passed it to sessions.js during wireup then it would be able to directly reference the store and make assertions against it directly.