input: Write a python function to count the frequency of each word in a given text.
  assert word_frequency("The quick brown fox jumps over the lazy dog") == {'The': 1, 'quick': 1, 'brown': 1, 'fox': 1, 'jumps': 1, 'over': 1, 'the': 1, 'lazy': 1, 'dog': 1}
  assert word_frequency("hello world hello") == {'hello': 2, 'world': 1}
  assert word_frequency("test test test test") == {'test': 4}
output: def word_frequency(text):
      words = text.split()
      frequency = {}
      for word in words:
          if word in frequency:
              frequency[word] += 1
          else:
              frequency[word] = 1
      return frequency