[tests] Add network_thread_ utility functions.

Add network thread_start(), network_thread_running() and
network_thread_join() utility functions in mininode.py and use
network_thread_running() in network thread assertions.

test/functional/test_framework/mininode.py

@@ -18,7 +18,7 @@ import logging
 import socket
 import struct
 import sys
-from threading import RLock, Thread
+import threading
 
 from test_framework.messages import *
 from test_framework.util import wait_until
@@ -397,9 +397,12 @@ mininode_socket_map = dict()
 # and whenever adding anything to the send buffer (in send_message()).  This
 # lock should be acquired in the thread running the test logic to synchronize
 # access to any data shared with the P2PInterface or P2PConnection.
-mininode_lock = RLock()
+mininode_lock = threading.RLock()
+
+class NetworkThread(threading.Thread):
+    def __init__(self):
+        super().__init__(name="NetworkThread")
 
-class NetworkThread(Thread):
     def run(self):
         while mininode_socket_map:
             # We check for whether to disconnect outside of the asyncore
@@ -412,3 +415,21 @@ class NetworkThread(Thread):
             [obj.handle_close() for obj in disconnected]
             asyncore.loop(0.1, use_poll=True, map=mininode_socket_map, count=1)
         logger.debug("Network thread closing")
+
+def network_thread_start():
+    """Start the network thread."""
+    NetworkThread().start()
+
+def network_thread_running():
+    """Return whether the network thread is running."""
+    return any([thread.name == "NetworkThread" for thread in threading.enumerate()])
+
+def network_thread_join(timeout=10):
+    """Wait timeout seconds for the network thread to terminate.
+
+    Throw if the network thread doesn't terminate in timeout seconds."""
+    network_threads = [thread for thread in threading.enumerate() if thread.name == "NetworkThread"]
+    assert len(network_threads) <= 1
+    for thread in network_threads:
+        thread.join(timeout)
+        assert not thread.is_alive()