input: Write a python function to count the frequency of words in a given string.
  assert count_word_frequency("this is a test string") == {'this': 1, 'is': 1, 'a': 1, 'test': 1, 'string': 1}
  assert count_word_frequency("test this is a test") == {'test': 2, 'this': 1, 'is': 1, 'a': 1}
  assert count_word_frequency("hello hello world") == {'hello': 2, 'world': 1}
output: def count_word_frequency(s):
      words = s.split()
      frequency = {}
      for word in words:
          if word in frequency:
              frequency[word] += 1
          else:
              frequency[word] = 1
      return frequency