message: Fix WhitelistController unit test teacher acquisition
  It was completely random and thus the existing unit tests would
  erroneously break in nondeterministic fashion once the vulnerabilities
  are closed (i.e. once only authorized users are allowed).
diff: @@ -18,8 +18,8 @@ describe('Whitelist', () => {
  describe(`GET ${BASE_URL}`, () => {
  it('should get a whitelist user', async () => {
  - const teacher = await FixtureUtils.getRandomTeacher();
  const course: ICourse = await FixtureUtils.getRandomCourse();
  + const teacher = await FixtureUtils.getRandomTeacherForCourse(course);
  const newWhitelistUser: IWhitelistUser = new WhitelistUser({
  firstName: 'Max',
  lastName: 'Mustermann',
  @@ -44,7 +44,7 @@ describe('Whitelist', () => {
  uid: '1236456',
  courseId: course._id
  };
  - const teacher = await FixtureUtils.getRandomTeacher();
  + const teacher = await FixtureUtils.getRandomTeacherForCourse(course);
  const res = await testHelper.commonUserPostRequest(teacher, '', whitelistUser);
  res.status.should.be.equal(200);
  res.body.firstName.should.be.equal(whitelistUser.firstName.toLowerCase());
  @@ -69,8 +69,8 @@ describe('Whitelist', () => {
  });
  it('should add an user by synchronizing', async () => {
  - const teacher = await FixtureUtils.getRandomTeacher();
  const course: ICourse = await FixtureUtils.getRandomCourse();
  + const teacher = await FixtureUtils.getRandomTeacherForCourse(course);
  const user: IUser = await User.create(
  { uid: '1236456',
  password: 'test1234',
  @@ -138,8 +138,8 @@ describe('Whitelist', () => {
  describe(`DELETE ${BASE_URL}`, () => {
  it('should delete a whitelist user', async () => {
  - const teacher = await FixtureUtils.getRandomTeacher();
  const course: ICourse = await FixtureUtils.getRandomCourse();
  + const teacher = await FixtureUtils.getRandomTeacherForCourse(course);
  const newWhitelistUser: IWhitelistUser = new WhitelistUser({
  firstName: 'Max',
  lastName: 'Mustermann',
  @@ -168,8 +168,8 @@ describe('Whitelist', () => {
  });
  it('should delete an user by synchronizing', async () => {
  - const teacher = await FixtureUtils.getRandomTeacher();
  const course: ICourse = await FixtureUtils.getRandomCourse();
  + const teacher = await FixtureUtils.getRandomTeacherForCourse(course);
  const member = course.students[0];
  const newWhitelistUser: IWhitelistUser = new WhitelistUser({
  firstName: member.profile.firstName,
  