input: Write a function to find the most frequent word in a given string.
  assert most_frequent("This is a test test string") == "test"
  assert most_frequent("apple orange apple banana orange apple") == "apple"
  assert most_frequent("one two two three three three four four four four") == "four"
output: def most_frequent(s):
      from collections import Counter
      words = s.split()
      freq = Counter(words)
      most_common_word, _ = freq.most_common(1)[0]
      return most_common_word