[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/importmulti.py

@@ -429,7 +429,7 @@ class ImportMultiTest (BitcoinTestFramework):
 
         # restart nodes to check for proper serialization/deserialization of watch only address
         self.stop_nodes()
-        self.nodes = self.start_nodes(2, self.options.tmpdir)
+        self.start_nodes()
         address_assert = self.nodes[1].validateaddress(watchonly_address)
         assert_equal(address_assert['iswatchonly'], True)
         assert_equal(address_assert['ismine'], False)