TestOnly: add setEthernetEnable support to EthernetManagerTest

Also fix the bug in teardown that setIncludeTestInterfaces(false)
should be called after the interfaces are destroyed, otherwise
the call to ifaceListener.eventuallyExpect(ABSENT) may receive
callbacks out of order (and it doesn't actually wait on the
interface being removed properly).

Test: atest EthernetManagerTest

Change-Id: I6650c028c121d932f10219d04d40d1ad60d9d4d8