ethernet: add test for tethered interface callbacks am: bd7a9c2596 am: 759c19335d

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

Change-Id: I682f6192bed760e526a1b1d5260546745a3455ef
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>