Function add_default_route 

pub async fn add_default_route<'a, I>(
    realm: &'a TestRealm<'a>,
    if_id: u64,
    route_set: &<I::RouteSetMarker as ProtocolMarker>::Proxy,
) -> bool
where
    I: Ip + FidlRouteAdminIpExt + FidlRouteIpExt,

Add a default route to the provided interface by id. Must not be called more than once for the same interface.

Returns whether the route was added to the RouteSet.