Write unit tests for the following functions
def add(x, y):
"""Add numbers"""
return x + y
def substract(x, y):
"""Substract numbers"""
return x - y
def multiply(x, y):
"""Multiply numbers"""
return x * y
def divide(x, y):
"""Divide numbers"""
if y == 0:
raise ValueError('Can not divide by zero!')
return x / y