input: Write a python function to generate a random string of specified length using lowercase letters.
  assert len(random_string(5)) == 5
  assert len(random_string(10)) == 10
  assert all(c in string.ascii_lowercase for c in random_string(15))
output: import random
  import string
  
  def random_string(length):
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))