tests/test_lightningd.py: move port into node.

We're about to change the JSONRPC, so let's put an explicit 'port' into
our node class.

We initialize it at startup time: in future I hope to use ephemeral ports
to make our tests more easily parallelizable.

Suggested-by: @cdecker
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/utils.py

@@ -367,6 +367,10 @@ class LightningNode(object):
         c.close()
         db.close()
 
+    def start(self):
+        self.daemon.start()
+        self.port = self.rpc.getinfo()['port']
+
     def stop(self, timeout=10):
         """ Attempt to do a clean shutdown, but kill if it hangs
         """
@@ -403,7 +407,7 @@ class LightningNode(object):
         else:
             self.daemon.stop()
 
-        self.daemon.start()
+        self.start()
 
     def fund_channel(self, l2, amount):