ethernet: ignore restricted interfaces in tests

This makes running interface callbacks tests easier on cuttlefish, where
a restricted interface is available.
In the future, it probably makes sense to add some tests that
specifically tests restricted interfaces. There have been a couple of
bugs in this area, so I will add test coverage when I get around to
fixing them.

Test: atest EthernetManagerTest
Change-Id: Ia29164cc910a85969a59aa67cdab3a55f52a2e09