tests/sys/net/routing: XFAIL the two failing tests
They have been failing for 1.5 months and the patch to fix them is stuck
in review so mark them as XFAIL for now to get Jenkins back to green.
To be reverted when https://reviews.freebsd.org/D28886 (or similar) is
commited.
Reviewed By: kp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29528