[Test Coverage] Offline payments methods

[naydav]

• Create tests for payment methods: Check/Money Order, Bank Transfer, Cash on Delivery, Purchase Order

Testcases should be introduced:
\Magento\GraphQl\Quote\Customer\SetOfflinePaymentMethodsOnCartTest
\Magento\GraphQl\Quote\Guest\SetOfflinePaymentMethodsOnCartTest

Example:
https://github.com/magento/graphql-ce/blob/2.3-develop/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer/SetOfflineShippingMethodsOnCartTest.php

https://github.com/magento/graphql-ce/blob/2.3-develop/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer/SetPaymentMethodOnCartTest.php

[naydav]

#632