bpo-41739: Fix test_logging.test_race_between_set_target_and_flush() (GH-22655)

The test now waits until all threads complete to avoid leaking
running threads.

Also, use regular threads rather than daemon threads.

Lib/test/test_logging.py

@@ -1164,22 +1164,27 @@ class MemoryHandlerTest(BaseTest):
         class MockRaceConditionHandler:
             def __init__(self, mem_hdlr):
                 self.mem_hdlr = mem_hdlr
+                self.threads = []
 
             def removeTarget(self):
                 self.mem_hdlr.setTarget(None)
 
             def handle(self, msg):
-                t = threading.Thread(target=self.removeTarget)
-                t.daemon = True
-                t.start()
+                thread = threading.Thread(target=self.removeTarget)
+                self.threads.append(thread)
+                thread.start()
 
         target = MockRaceConditionHandler(self.mem_hdlr)
+        try:
             self.mem_hdlr.setTarget(target)
 
             for _ in range(10):
                 time.sleep(0.005)
                 self.mem_logger.info("not flushed")
                 self.mem_logger.warning("flushed")
+        finally:
+            for thread in target.threads:
+                threading_helper.join_thread(thread)
 
 
 class ExceptionFormatter(logging.Formatter):

Misc/NEWS.d/next/Tests/2020-10-12-00-11-47.bpo-41739.wSCc4K.rst

@@ -0,0 +1,2 @@
+Fix test_logging.test_race_between_set_target_and_flush(): the test now
+waits until all threads complete to avoid leaking running threads.