add mock and api-mock utils file and example

[gabbork]

Add python mock example and utils file like this one

 # tests/utils.py
  1 import io                                                                                                                 
  2 import json
  3 
  4 
  5 def get_fake_file(filename):
  6     with open(f'tests/data/{filename}', encoding='utf-8') as data_file:
  7         output = io.StringIO(data_file.read())
  8         return output
  9 
 10 
 11 def get_fake_json(filename):
 12     with open(f'tests/data/{filename}', encoding='utf-8') as data_file:
 13         return json.loads(data_file.read())

It's not directly related with BDD but very common and useful.
We also should add a Documentation section describing usage and examples.