[qa] Remove hardcoded "4 nodes" from test_framework

qa/rpc-tests/invalidateblock.py

@@ -13,10 +13,11 @@ from test_framework.util import *
 class InvalidateTest(BitcoinTestFramework):
     
         
-    def setup_chain(self):
-        print("Initializing test directory "+self.options.tmpdir)
-        initialize_chain_clean(self.options.tmpdir, 3)
-                 
+    def __init__(self):
+        super().__init__()
+        self.setup_clean_chain = True
+        self.num_nodes = 3
+
     def setup_network(self):
         self.nodes = []
         self.is_network_split = False