[IMP] point_of_sale: Adapt tests to work with/without demo data

addons/point_of_sale/models/account_bank_statement.py

addons/point_of_sale/models/pos_config.py

addons/point_of_sale/tests/common.py

addons/point_of_sale/tests/test_anglo_saxon.py

addons/point_of_sale/tests/test_frontend.py

addons/point_of_sale/tests/test_point_of_sale_flow.py

addons/point_of_sale/tests/test_point_of_sale_ui.py

addons/point_of_sale/tests/test_pos_basic_config.py

...nt_of_sale/tests/test_pos_multiple_receivable_accounts.py

...ns/point_of_sale/tests/test_pos_multiple_sale_accounts.py

addons/point_of_sale/tests/test_pos_other_currency_config.py

addons/point_of_sale/tests/test_pos_products_with_tax.py

addons/point_of_sale/tests/test_pos_setup.py

addons/point_of_sale/tests/test_pos_stock_account.py

addons/point_of_sale/tests/test_pos_with_fiscal_position.py