[Test coverage] testReSetShippingMethod

[naydav]

Query scheme: app/code/Magento/QuoteGraphQl/etc/schema.graphqls

Need to add testReSetPaymentMethod case to

\Magento\GraphQl\Quote\Customer\SetShippingMethodsOnCartTest
\Magento\GraphQl\Quote\Guest\SetShippingMethodsOnCartTest

Functionality should be fixed if needed.

Please, reuse current fixtures instead of creating new.
Example: https://github.com/magento/graphql-ce/pull/471/files

More 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]

#496