input: Write a python function to find all possible substrings of a given string.
  assert find_substrings("abc") == ['a', 'ab', 'abc', 'b', 'bc', 'c']
  assert find_substrings("fun") == ['f', 'fu', 'fun', 'u', 'un', 'n']
  assert find_substrings("test") == ['t', 'te', 'tes', 'test', 'e', 'es', 'est', 's', 'st', 't']
output: def find_substrings(s):
      substrings = []
      for i in range(len(s)):
          for j in range(i + 1, len(s) + 1):
              substrings.append(s[i:j])
      return substrings