[Test Coverage] 'SetShippingMethodsOnCart' functionality

[TomashKhamlai]

Need to cover SetShippingMethodsOnCart functionality with API-functional tests both for Customer and Guest. Functionality should be fixed if needed.

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

Test Cases:

\Magento\GraphQl\Quote\Customer\SetShippingMethodsOnCartTest
- testSetShippingMethodIfMethodIsDisabled

\Magento\GraphQl\Quote\Guest\SetShippingMethodsOnCartTest
- testSetShippingMethodIfMethodIsDisabled

Please, reuse current fixtures (from the same class) instead of creating new.

[naydav]

#582