Merge bitcoin/bitcoin#33140: test: Avoid shutdown race in NetworkThread

fa6db79 test: Avoid shutdown race in NetworkThread (MarcoFalke)

Pull request description:

  Locally, I am seeing rare intermittent exceptions in the network thread:

  ```
  stderr:
  Exception in thread NetworkThread:
  Traceback (most recent call last):
  File "/python3.10/threading.py", line 1016, in _bootstrap_inner
  self.run()
  File "./test/functional/test_framework/p2p.py", line 744, in run
  self.network_event_loop.run_forever()
  File "/python3.10/asyncio/base_events.py", line 603, in run_forever
  self._run_once()
  File "/python3.10/asyncio/base_events.py", line 1871, in _run_once
  event_list = self._selector.select(timeout)
  AttributeError: 'NoneType' object has no attribute 'select'
  ```

  I can reproduce this intermittently via `while ./bld-cmake/test/functional/test_runner.py $(for i in {1..400}; do echo -n "tool_rpcauth "; done)  -j 400 ; do true ; done`.

  I suspect this is a race where the shutdown starts the close of the network thread while it is starting.

  A different exception showing this race can be reproduced via:

  ```diff
  diff --git a/test/functional/test_framework/p2p.py b/test/functional/test_framework/p2p.py
  index 610aa4c..64561e157c 100755
  --- a/test/functional/test_framework/p2p.py
  +++ b/test/functional/test_framework/p2p.py
  @@ -741,6 +741,7 @@ class NetworkThread(threading.Thread):

       def run(self):
           """Start the network thread."""
  +        import time;time.sleep(.1)
           self.network_event_loop.run_forever()

       def close(self, *, timeout=10):
  ```

  It is trivial to reproduce via any test (e.g. `./bld-cmake/test/functional/tool_rpcauth.py`) and shows a similar traceback to the one above:

  ```
  Exception in thread NetworkThread:
  Traceback (most recent call last):
    File "/python3.10/threading.py", line 1016, in _bootstrap_inner
      self.run()
    File "./test/functional/test_framework/p2p.py", line 745, in run
      self.network_event_loop.run_forever()
    File "/python3.10/asyncio/base_events.py", line 591, in run_forever
      self._check_closed()
    File "/python3.10/asyncio/base_events.py", line 515, in _check_closed
      raise RuntimeError('Event loop is closed')
  RuntimeError: Event loop is closed
  ```

  So fix the second runtime error in hope of fixing the first one as well.

ACKs for top commit:
  brunoerg:
    code review ACK fa6db79

Tree-SHA512: ca352ebf7929456ea2bbfcfe4f726adcbfcfb3dc0edeaddae7f6926f998888f0bd8b987ddef60308266eeab6bffa7ebdc32f5908db9de5404df95635dae4a8f6

Author: fanquake

test/functional/test_framework/test_framework.py

@@ -258,6 +258,7 @@ def setup(self):
         self.log.debug('Setting up network thread')
         self.network_thread = NetworkThread()
         self.network_thread.start()
+        self.wait_until(lambda: self.network_thread.network_event_loop.is_running())
 
         if self.options.usecli:
             if not self.supports_cli: