tests: Fix crash in test_mp_attr

tests: Fix crash in test_mp_attr

Some recent changes in BGP now require that the
peer's nexthop have a valid ifp when we are looking
at:

	case BGP_ATTR_NHLEN_IPV6_GLOBAL_AND_LL:
	case BGP_ATTR_NHLEN_VPNV6_GLOBAL_AND_LL:

This assumption makes sense for this type of Nexthop Attribute.
So for the test let's jimmy up a `fake` enough interface pointer
so that the actual test we can focus on what we are actually
testing.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>