Introduce mock for USPS shipping testing

[lenaorobei]

Description (*)

Instead of real calls to the carrier service we need to introduce mock object that will return expected info.

Additional information

1. Introduce new test module under dev/tests/api-functional/_files/Magento.
2. Module will serve for other carriers as well.
3. Declare preference in di.xml for Magento\Usps\Model\Carrier.
4. Instead of real call to the gateway, return expected info.
5. Undo skipped tests Magento\GraphQl\Usps\SetUspsShippingMethodsOnCartTest [Shipping methods] Test coverage of USPS shipping method #662.

[naydav]

#753