input: Write a python function to generate a random string of specified length using lowercase letters from the English alphabet.
  assert len(gen_random_string(5)) == 5
  assert len(gen_random_string(8)) == 8
  assert len(gen_random_string(12)) == 12
output: import random
  import string
  
  def gen_random_string(length):
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))