input: Write a function to filter strings that do not contain the substring "bad".
  assert filter_strings(["good","weird","great","amazing"])==["good","weird","great","amazing"]
  assert filter_strings(["good","bad","weird","badgreat","amazing"])==["good","weird","amazing"]
  assert filter_strings(["badgood","baddest","goodest","bad","good"])==["goodest","good"]
output: def filter_strings(strings):
      return [s for s in strings if "bad" not in s]