selftests: fib_tests: Add test cases for IPv4/IPv6 in route notify

Add tests to check whether the total fib info length is calculated
corretly in route notify process.

Signed-off-by: Lu Wei <luwei32@huawei.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230222083629.335683-3-luwei32@huawei.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>