input: Write a python function to filter a list of strings based on a substring.
  assert filter_list(["cat", "dog", "elephant", "rabbit", "duck"], "ra") == ["rabbit"]
  assert filter_list(["apple", "banana", "grape", "kiwi", "orange"], "ap") == ["apple", "grape"]
  assert filter_list(["sun", "moon", "star", "planet"], "ar") == ["star"]
output: def filter_list(strings, substring):
      return [s for s in strings if substring in s]