input: Write a function to filter strings that contain a specific substring.
  assert filter_strings(["Hello", "World", "Hey"], "or") == ["World"]
  assert filter_strings(["apple", "banana", "apricot", "cherry"], "ap") == ["apple", "apricot"]
  assert filter_strings(["dog", "cat", "hamster", "fish"], "cat") == ["cat"]
output: def filter_strings(strings, substring):
      return [s for s in strings if substring in s]