pytest: Configure logging in a fixture to match stdout capturing

pytest captures the output by monkey patching out `sys.stdout`. This may
conflict with our use of `sys.stdout` when configuring logging, resulting in
the "Write to closed file" issue that is spamming the logs. By making the
logging configuration a fixture hopefully we always use the correct
stdout (after pytest has monkey-patched it).

contrib/pyln-testing/pyln/testing/utils.py

@@ -17,7 +17,6 @@ import sqlite3
 import string
 import struct
 import subprocess
-import sys
 import threading
 import time
 
@@ -69,10 +68,6 @@ SLOW_MACHINE = env("SLOW_MACHINE", "0") == "1"
 TIMEOUT = int(env("TIMEOUT", 180 if SLOW_MACHINE else 60))
 
 
-if TEST_DEBUG:
-    logging.basicConfig(level=logging.DEBUG, stream=sys.stdout)
-
-
 def wait_for(success, timeout=TIMEOUT):
     start_time = time.time()
     interval = 0.25