input: Write a python function to count the occurrence of each character in a string.
  assert count_characters("hello") == {'h': 1, 'e': 1, 'l': 2, 'o': 1}
  assert count_characters("test") == {'t': 2, 'e': 1, 's': 1}
  assert count_characters("character") == {'c': 2, 'h': 1, 'a': 2, 'r': 2, 't': 1, 'e': 1}
output: def count_characters(s):
      count_dict = {}
      for char in s:
          if char in count_dict:
              count_dict[char] += 1
          else:
              count_dict[char] = 1
      return count_dict