message: Fix teacher POST /api/download/pdf/individual test
  ...by fusing it with the "(course admin)" variant,
  using the FixtureUtils.getRandomTeacherForCourse.
  (The "(teacher)" variant was broken because the course.teachers
  currently is not guaranteed to contain any teachers, since the
  FixtureLoader assigns 0 to 2 teachers randomly per course.)
diff: @@ -180,9 +180,9 @@ describe('DownloadFile', () => {
  .catch(err => err.response);
  res.status.should.be.equal(500);
  });
  - it('should pass (course admin)', async () => {
  + it('should pass (teacher)', async () => {
  const course = await FixtureUtils.getRandomCourseWithAllUnitTypes();
  - const teacher = await User.findById(course.courseAdmin);
  + const teacher = await FixtureUtils.getRandomTeacherForCourse(course);
  const testData = await prepareTestData(course);
  const res = await chai.request(app)
  .post(BASE_URL + '/pdf/individual')
  @@ -204,19 +204,6 @@ describe('DownloadFile', () => {
  expect(res).to.have.status(200);
  expect(res).to.be.json;
  }).timeout(30000);
  - // For some reason there is no teacher for a course.
  - it('should pass (teacher)', async () => {
  - const course = await FixtureUtils.getRandomCourseWithAllUnitTypes();
  - const teacher = await User.findById(course.teachers[0]);
  - const testData = await prepareTestData(course);
  - const res = await chai.request(app)
  - .post(BASE_URL + '/pdf/individual')
  - .set('Cookie', `token=${JwtUtils.generateToken(teacher)}`)
  - .send(testData)
  - .catch(err => err.response);
  - expect(res).to.have.status(200);
  - expect(res).to.be.json;
  - }).timeout(30000);
  });
  describe(`POST ${BASE_URL + '/pdf/single'}`, () => {
  