Fix EthernetTetheringTest#testLocalOnlyTethering flaky

The testLocalOnlyTethering only verifies whether an IPv6 RA
with a ULA prefix is sent out. If the test interface is killed
after the RA is sent out while the DHCP server is starting,
the error callback with exception is raised. RunDhcp
verification is used to ensure that DHCP server is started.

Bug: 286701571
Test: TH

Change-Id: Iccebcedbba7b2f9cd28bb1cc06af928ff8dfaaca
1 file changed