Define network.try_broadcasting() method.

Use it when rebroadcasting a force-close tx,
because the channel state is already set.

electrum/tests/test_lnpeer.py

@@ -64,7 +64,7 @@ class MockNetwork:
     def get_local_height(self):
         return 0
 
-    async def broadcast_transaction(self, tx):
+    async def try_broadcasting(self, tx, name):
         if self.tx_queue:
             await self.tx_queue.put(tx)