adjust tests for route metric of default route (th/platform-route-pt4)

Previously, NetworkManager would configure default routes via ip route replace. That meant, it needed to take care about not to configure the same dst/plen,metric route on multiple interfaces, because they would conflict and replace each other. That was solved by having a NMDefaultRouteManager which corrdinates configuring the default routes. In face of duplicate routes, it would then bump the metric to avoid the conflict.

Now, we use instead ip route append and each interface can hold conflicting routes just fine. That makes the bumping of the route no longer necessary. Adjust tests for that.

This adjusts the tests for https://github.com/NetworkManager/NetworkManager/pull/26 . This is already merged on master, so this fixes expected 3 failures there.