Also skip sys/net/if_lagg_test:witness on non-i386

The LOR also happens on amd64 and other architectures. Ideally we would
fix this. However, in order to get Jenkins green again to catch real
regressions, we should skip this test for now.

PR: 251726
Reviewed By: lwhsu
Differential Revision: https://reviews.freebsd.org/D29341