Move setupOnce from EthernetTetheringTestBase to EthernetTetheringTest

If we want to get rid of the flakiness, we may need to change the
production code. You can find a more detailed analysis in
b/318121782/comment#4.

Here's a quick and easy alternative fix: just move setupOnce from
EthernetTetheringTestBase to EthernetTetheringTest. The flakiness
started when I added setupOnce into EthernetTetheringTestBase to
fix flakiness in EthernetTetheringTest.

Bug: 318121782
Test: TH
Change-Id: I0a6fb25773aa60872f71fe4b34e42a0b745a767f
2 files changed