input: Write a function to generate a random string of a given length using lowercase letters. The function should take an integer parameter `length` and return a string of that length.
  assert random_string(6)=='aaaaaa'
  assert random_string(6)=='bbbbbb'
  assert random_string(6)=='cccccc'
  assert random_string(6)=='dddddd'
  assert random_string(6)=='eeeeee'
output: import random
  import string
  
  def random_string(length):
      return ''.join(random.choices(string.ascii_lowercase, k=length))