input: Write a python function to generate a random string of lowercase letters with a given length.
  assert len(random_string(6)) == 6
  assert len(random_string(10)) == 10
  assert isinstance(random_string(5), str)
output: import random
  import string
  
  def random_string(length):
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))