[Test Coverage] Get Specified shipping address

[naydav]

• Test coverage for "Get Specified shipping address"

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

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

[atwixfirster]

Pull Request: https://github.com/magento/graphql-ce/pull/652/files