selftests: net: local_termination: annotate the expected failures

Vladimir said when adding this test:

  The bridge driver fares particularly badly [...] mainly because
  it does not implement IFF_UNICAST_FLT.

See commit 90b9566a

 ("selftests: forwarding: add a test for
local_termination.sh").

We don't want to hide the known gaps, but having a test which
always fails prevents us from catching regressions. Report
the cases we know may fail as XFAIL.

Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20240516152513.1115270-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>