Start Ethernet tethering test once before testing

The first tethering test case might have a tethering restart when
handling IP conflicts. Currently, we start a hotspot once before
tethering, but the test is still flaky in Cuttlefish because it
doesn't support hotspots. Instead, we'll start ethernet tethering.

Bug: 254183718
Test: atest CtsTetheringTest
Change-Id: I695d4e1b3f961893bc30b3144faf7ad7d2d70574