[REF] point_of_sale: converted yml test cases to python unit test

addons/point_of_sale/__openerp__.py

addons/point_of_sale/test/00_register_open.yml

addons/point_of_sale/test/01_order_to_payment.yml

addons/point_of_sale/test/02_order_to_invoice.yml

addons/point_of_sale/test/account_statement_reports.yml

addons/point_of_sale/test/tests_before.xml

addons/point_of_sale/tests/__init__.py

addons/point_of_sale/tests/common.py

addons/point_of_sale/tests/test_point_of_sale_flow.py