input: Write a function to check if the user input is an integer.
  assert check_user_input("123") == "You have entered an integer."
  assert check_user_input("abc") == "Invalid input."
  assert check_user_input("456") == "You have entered an integer."
output: def check_user_input(user_input):
      if user_input.isdigit():
          return "You have entered an integer."
      else:
          return "Invalid input."