public void Test_NullStudentCantJoinTheCourse()
        {
            List<Student> students = new List<Student>();
            for (int i = 0; i < 10; i++)
            {
                students.Add(new Student("Student" + i));
            }

            Course someCourse = new Course(students);
            someCourse.Join(null);
        }
        public void Test_ThirtyFirstStudentCantJoinTheCourse()
        {
            List<Student> students = new List<Student>();
            for (int i = 0; i < 30; i++)
            {
                students.Add(new Student("Student" + i));
            }

            Course someCourse = new Course(students);
            someCourse.Join(new Student("Pesho"));
        }