selftests: add new tests for vxlan vnifiltering

This patch adds a new test script test_vxlan_vnifiltering.sh
with tests for vni filtering api, various datapath tests.
Also has a test with a mix of traditional, metadata and vni
filtering devices inuse at the same time.

Signed-off-by: Roopa Prabhu <roopa@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>