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