[tests] Functional tests must explicitly set num_nodes

test/functional/wallet.py

@@ -8,6 +8,7 @@ from test_framework.util import *
 
 class WalletTest(BitcoinTestFramework):
     def set_test_params(self):
+        self.num_nodes = 4
         self.setup_clean_chain = True
         self.extra_args = [['-usehd={:d}'.format(i%2==0)] for i in range(4)]