selftests: netfilter: move fib vrf test to nft_fib.sh

It was located in conntrack_vrf.sh because that already had the VRF bits.
Lets not add to this and move it to nft_fib.sh where this belongs.

No functional changes for the subtest intended.
The subtest is limited, it only covered 'fib oif'
(route output interface query) when the incoming interface is part
of a VRF.

Next we can extend it to cover 'fib type' for VRFs and also check fib
results when there is an unrelated VRF in same netns.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>