Move assert num_nodes is set into main()

This allows a BitcoinTestFramework child class to set test parameters in an
overridden setup() rather than in an overridden set_test_params().

test/functional/test_framework/test_framework.py

@@ -100,11 +100,11 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
         self.bind_to_localhost_only = True
         self.set_test_params()
 
-        assert hasattr(self, "num_nodes"), "Test must set self.num_nodes in set_test_params()"
-
     def main(self):
         """Main function. This should not be overridden by the subclass test scripts."""
 
+        assert hasattr(self, "num_nodes"), "Test must set self.num_nodes in set_test_params()"
+
         self.parse_args()
 
         try: