input: Write a python function to count the five most common words in a given text.
  assert count_words('hello world hello hello world') == [('hello', 3), ('world', 2)]
  assert count_words('test test test testing testing one two three') == [('test', 3), ('testing', 2), ('one', 1), ('two', 1), ('three', 1)]
  assert count_words('a a a a b b b c c d') == [('a', 4), ('b', 3), ('c', 2), ('d', 1)]
output: def count_words(text):
      from collections import Counter
      words = text.split()
      word_counts = Counter(words)
      return word_counts.most_common(5)