Test coverage: testReSetPaymentMethod

[naydav]

Need to add testReSetPaymentMethod case to:

\Magento\GraphQl\Quote\Customer\SetPaymentMethodOnCartTest
\Magento\GraphQl\Quote\Guest\SetPaymentMethodOnCartTest

Examples you could find in
https://github.com/magento/graphql-ce/tree/2.3-develop/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer

https://github.com/magento/graphql-ce/tree/2.3-develop/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Guest

[naydav]

#471