It’s not exactly a mock/stub, but you can use testplanet. This is a test utility used from many places in the Storj which can be used to start up a local Storj network (satellite, storagenode, everything) for the duration of the test. You can get the uplink reference from it and use it.
However, it’s not a mock, but a real ephemeral test network. Quite fast, but it requires a running database in your test environment.