Add tests for nix-vector routing.
Summary of tests:
- Check the routing is possible with the shortest path.
- Test the routing is working
- Test the path
- Set down the nA interface on nA-nC channel.
- Check if the cache is cleared.
- Test the routing is working.
- Check the new path now.
- Set down the nC interface on nB-nC channel.
- Check that routing is not possible.
Signed-off-by: Ameya Deshpande email@example.com