How do I prompt codex to generate unit tests for my code

Question

How do I prompt codex (code-davinci-002) model to generate all the unit tests for my code? I understand from some posts that I should rather do it function by function for better results.

So I tried

# generate unit tests for
def add (n1, n2):
    return n1 + n2

which gave

# generate unit tests for
def add (n1, n2):
    return n1 + n2

def subtract (n1, n2):
    return n1 - n2

def multiply (n1, n2):
    return n1 * n2

def divide (n1, n2):
    return n1 / n2

that sounds rather incorrect. Only once it gave correct result of

def test_add (n1, n2):

# with some correct statements with parameter. values

How do I ensure that it always gives correct results? Also how to make sure that it does thorough job (e.g. different types, 0 result, positive / negative results etc.)