input: Write a python function to count the number of strings that contain the substring "cat".
  assert count_substring_cat(["cat keeper", "cat lover", "lizard wrangler", "cat wrangler"]) == 3
  assert count_substring_cat(["dog keeper", "cat lover", "lizard wrangler", "cat wrangler"]) == 2
  assert count_substring_cat(["dog keeper", "dog lover", "lizard wrangler", "dog wrangler"]) == 0
output: def count_substring_cat(strings):
      return sum(1 for s in strings if 'cat' in s)