message: API: Use TestHelper in lecture unit test
diff: import * as chai from 'chai';
  -import {Server} from '../../src/server';
  -import {FixtureLoader} from '../../fixtures/FixtureLoader';
  -import {JwtUtils} from '../../src/security/JwtUtils';
  +import {TestHelper} from '../TestHelper';
  import {User} from '../../src/models/User';
  import {Course} from '../../src/models/Course';
  import {FixtureUtils} from '../../fixtures/FixtureUtils';
  @@ -10,14 +8,12 @@ import {Lecture} from '../../src/models/Lecture';
  chai.use(chaiHttp);
  const should = chai.should();
  -const app = new Server().app;
  const BASE_URL = '/api/lecture';
  -const fixtureLoader = new FixtureLoader();
  +const testHelper = new TestHelper(BASE_URL);
  describe('Lecture', () => {
  - // Before each test we reset the database
  beforeEach(async () => {
  - await fixtureLoader.load();
  + await testHelper.resetForNextTest();
  });
  describe(`DELETE ${BASE_URL}` , () => {
  @@ -26,9 +22,7 @@ describe('Lecture', () => {
  const lectureId = await course.lectures[0];
  const courseAdmin = await User.findOne({_id: course.courseAdmin});
  - const res = await chai.request(app)
  - .del(BASE_URL + '/' + lectureId)
  - .set('Cookie', `token=${JwtUtils.generateToken(courseAdmin)}`);
  + const res = await testHelper.commonUserDeleteRequest(courseAdmin, `/${lectureId}`);
  res.status.should.be.equal(200);
  const courseWithDeletedLecture = await Course.findById(course._id);
  