selftests: net: initial fib rule tests

This adds a first set of tests for fib rule match/action for
ipv4 and ipv6. Initial tests only cover action lookup table.
can be extended to cover other actions in the future.
Uses ip route get to validate the rule lookup.

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