Add SkipPresubmit to EthernetManagerTest

Each method is 34~46% flaky, so tests are failing most changes
in presubmit.
Add SkipPresubmit to avoid blocking all changes. This should be
removed after fixing the flakes as soon as possible, to avoid
regressions being added to the tests in the meantime.

Bug: 240323229
Bug: 241337236
Change-Id: Ie92ed9343a776c66e6942a535102e074d3f692c8
Test: TH
1 file changed