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