Add an Ethernet tethering test.

This test uses EthernetManager's ability to use test network
interfaces to test tethering. This provides a fairly realistic
integration test for Tethering and its callbacks, IpServer and
DhcpServer, and so on.

It is in a new integration/ directory under Tethering because I
didn't really know where to put it. It's fast enough to run in
presubmit, but it didn't seem to be appropriate to call it a
unit test, and in the future we could also use this test to do
some limited testing of real Ethernet tethering as well.

Bug: 150644681
Test: atest TetheringIntegrationTests:EthernetTetheringTest --rerun-until-failure 100
Change-Id: Ifcda70b73848b1fd4c26b031e53f0a6500cc93d4