Add testTetheringUpstream cts test

If cellular is the mobile network, tethering should select it as
upstream. If DUN is required, tethering should use DUN as its upstream.

Bug: 153614366
Bug: 150644305
Test: atest CtsTetheringTest

Change-Id: Ief9c745322e8705c0e50185e749a81c5a9d20743
2 files changed
tree: f8bf01f88b297963f10f9cf8b7b9326a4740638b
  1. tests/