selftests/net: test nexthop without gw

This test implement the scenario described in the commit
"ip: fix dflt addr selection for connected nexthop".
The test configures a nexthop object with an output device only (no gateway
address) and a route that uses this nexthop. The goal is to check if the
kernel selects a valid source address.

Link: https://lore.kernel.org/netdev/20220712095545.10947-1-nicolas.dichtel@6wind.com/
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20220713114853.29406-2-nicolas.dichtel@6wind.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>