When forwarding is on, DadProxyTest fails because forwarding
disables optimistic addresses. This results in the DAD proxy
getting EADDRNOTAVAIL.
Forwarding is on quite often on real hardware because many
solutions for wifi calling use forwarding.
Fix the test to retry after two seconds if the packet is not
forwarded on the first attempt.
It would also be possible to make this work more reliably in the
test by simply disabling DAD or disabling forwarding. However,
because DAD does happen in real usage (downstream interfaces
always disable DAD, but upstream interfaces don't), that seems
risky. For example, if the test disabled DAD, it would not catch
bugs where the DAD proxy crashed or stopped forwarding if it got
EADDRNOTAVAIL.
Test: atest TetheringPrivilegedTests on coral, which has forwarding on
Change-Id: I58280ef7c0e40371cd770ead4c8baa7190c288fd
b6bffbca95