[tests] Update start/stop node functions to be private module functions

This commit marks the start/stop functions in util.py as private module
functions. A future PR will remove these entirely and move the
functionality directly into the BitcoinTestFramework class, but setting them as
private in this PR will prevent anyone from accidentally calling them
before that future PR is merged.

test/functional/test_framework/test_framework.py

@@ -31,10 +31,10 @@ from .util import (
     p2p_port,
     rpc_url,
     set_node_times,
-    start_node,
-    start_nodes,
-    stop_node,
-    stop_nodes,
+    _start_node,
+    _start_nodes,
+    _stop_node,
+    _stop_nodes,
     sync_blocks,
     sync_mempools,
     wait_for_bitcoind_start,
@@ -91,7 +91,7 @@ class BitcoinTestFramework(object):
         extra_args = None
         if hasattr(self, "extra_args"):
             extra_args = self.extra_args
-        self.nodes = start_nodes(self.num_nodes, self.options.tmpdir, extra_args)
+        self.nodes = _start_nodes(self.num_nodes, self.options.tmpdir, extra_args)
 
     def run_test(self):
         raise NotImplementedError
@@ -194,16 +194,16 @@ class BitcoinTestFramework(object):
     # Public helper methods. These can be accessed by the subclass test scripts.
 
     def start_node(self, i, dirname, extra_args=None, rpchost=None, timewait=None, binary=None, stderr=None):
-        return start_node(i, dirname, extra_args, rpchost, timewait, binary, stderr)
+        return _start_node(i, dirname, extra_args, rpchost, timewait, binary, stderr)
 
     def start_nodes(self, num_nodes, dirname, extra_args=None, rpchost=None, timewait=None, binary=None):
-        return start_nodes(num_nodes, dirname, extra_args, rpchost, timewait, binary)
+        return _start_nodes(num_nodes, dirname, extra_args, rpchost, timewait, binary)
 
     def stop_node(self, num_node):
-        stop_node(self.nodes[num_node], num_node)
+        _stop_node(self.nodes[num_node], num_node)
 
     def stop_nodes(self):
-        stop_nodes(self.nodes)
+        _stop_nodes(self.nodes)
 
     def split_network(self):
         """