[tests] TestNode: separate add_node from start_node

Separates the act of creating a TestNode object from starting the node.
The test_framework now keeps track of its list of TestNodes, and test
writers can call start_node() and stop_node() without having to update
the self.nodes list.

test/functional/mempool_persist.py

@@ -63,9 +63,8 @@ class MempoolPersistTest(BitcoinTestFramework):
 
         self.log.debug("Stop-start node0 and node1. Verify that node0 has the transactions in its mempool and node1 does not.")
         self.stop_nodes()
-        self.nodes = []
-        self.nodes.append(self.start_node(0, self.options.tmpdir))
-        self.nodes.append(self.start_node(1, self.options.tmpdir))
+        self.start_node(0)
+        self.start_node(1)
         # Give bitcoind a second to reload the mempool
         time.sleep(1)
         wait_until(lambda: len(self.nodes[0].getrawmempool()) == 5)
@@ -73,16 +72,14 @@ class MempoolPersistTest(BitcoinTestFramework):
 
         self.log.debug("Stop-start node0 with -persistmempool=0. Verify that it doesn't load its mempool.dat file.")
         self.stop_nodes()
-        self.nodes = []
-        self.nodes.append(self.start_node(0, self.options.tmpdir, ["-persistmempool=0"]))
+        self.start_node(0, extra_args=["-persistmempool=0"])
         # Give bitcoind a second to reload the mempool
         time.sleep(1)
         assert_equal(len(self.nodes[0].getrawmempool()), 0)
 
         self.log.debug("Stop-start node0. Verify that it has the transactions in its mempool.")
         self.stop_nodes()
-        self.nodes = []
-        self.nodes.append(self.start_node(0, self.options.tmpdir))
+        self.start_node(0)
         wait_until(lambda: len(self.nodes[0].getrawmempool()) == 5)
 
 if __name__ == '__main__':