add hidden method setTestNetworkAsUpstream

`setTestNetworkAsUpstream` allows to specify a test network as the upstream of Thread network. This is going to be used in Thread integration tests. E.g. android.net.thread.BorderRoutingTest.

Bug: 295843010
Test: atest android.net.thread.BorderRoutingTest

Change-Id: I66474e2aa5b3434c4c66078a1d6bd6e522e32ae5
4 files changed