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
diff --git a/tests/cts/tethering/Android.bp b/tests/cts/tethering/Android.bp
index 9f32403..85bb0e0 100644
--- a/tests/cts/tethering/Android.bp
+++ b/tests/cts/tethering/Android.bp
@@ -28,6 +28,7 @@
         "TetheringCommonTests",
         "TetheringIntegrationTestsLib",
         "compatibility-device-util-axt",
+        "cts-net-utils",
         "net-tests-utils",
         "ctstestrunner-axt",
         "junit",