Add test for relay bandwidth capping.

Feature was added in
https://webrtc-review.googlesource.com/c/src/+/171226

Bug: webrtc:11434
Change-Id: Iee1e350976ab4043f15c5932cdc4f53b413bb302
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171861
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Christoffer Rodbro <crodbro@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30940}

rtc_base/network_route.h

@@ -46,6 +46,9 @@ class RouteEndpoint {
                          /* adapter_id = */ 0, network_id,
                          /* uses_turn = */ false);
   }
+  RouteEndpoint CreateWithTurn(bool uses_turn) const {
+    return RouteEndpoint(adapter_type_, adapter_id_, network_id_, uses_turn);
+  }
 
   AdapterType adapter_type() const { return adapter_type_; }
   uint16_t adapter_id() const { return adapter_id_; }