Write a course_grader function that takes a list of test scores as its parameter.
It will add up these test scores and calculate an average score.
It should then return a message of "pass" or "fail" depending on these two conditions:
- If the average score is greater than or equal to 70 and no single test score is below 50, then return a message of "pass".
- If the average score is lower than 70 or at least one test score is below 50, then return a message of "fail".