ethernet: add test for tethered interface callbacks am: bd7a9c2596

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/18727987

Change-Id: I341bed18b53471dbb26b01a27a01178ebc2229d9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>