input: Write a python function to count the number of occurrences of a substring in a given string.
  assert countStringOccurrences('Hello World, Hello World', 'Hello World') == 2
  assert countStringOccurrences('abababab', 'ab') == 4
  assert countStringOccurrences('aaaaaa', 'aa') == 5
output: def countStringOccurrences(s, sub):
      count = start = 0
      while start < len(s):
          start = s.find(sub, start) + 1
          if start > 0:
              count += 1
          else:
              break
      return count