input: Write a function to search for a substring within a string and return "Substring found!" if the substring is present, otherwise return "Substring not found."
  assert search_substring("hello world", "world") == "Substring found!"
  assert search_substring("python programming", "java") == "Substring not found."
  assert search_substring("openai", "ai") == "Substring found!"
  assert search_substring("data science", "science") == "Substring found!"
  assert search_substring("machine learning", "deep learning") == "Substring not found."
output: def search_substring(s, sub):
      if sub in s:
          return "Substring found!"
      else:
          return "Substring not found!"