input: Write a python function to concatenate a list of strings into a single string.
  assert concatenate_strings(['Hello', 'name', 'John']) == 'HellonameJohn'
  assert concatenate_strings(['Python', 'is', 'fun']) == 'Pythonisfun'
  assert concatenate_strings(['This', 'is', 'a', 'test']) == 'Thisisatest'
output: def concatenate_strings(strings):
      return ''.join(strings)