test: refactor functional tests to use restart_node

test/functional/p2p_segwit.py

@@ -1898,8 +1898,7 @@ class SegWitTest(BitcoinTestFramework):
     def test_upgrade_after_activation(self):
         """Test the behavior of starting up a segwit-aware node after the softfork has activated."""
 
-        self.stop_node(2)
-        self.start_node(2, extra_args=["-segwitheight={}".format(SEGWIT_HEIGHT)])
+        self.restart_node(2, extra_args=["-segwitheight={}".format(SEGWIT_HEIGHT)])
         connect_nodes(self.nodes[0], 2)
 
         # We reconnect more than 100 blocks, give it plenty of time