Fix error in test suite teardown

The error seen was:

Exception ignored in atexit callback: <bound method DeferredStreamHandler.flush of <DeferredStreamHandler <_io.FileIO [closed]> (WARNING)>>
Traceback (most recent call last):
  File "/home/dan/src/salt/salt/_logging/handlers.py", line 68, in flush
    super().flush()
  File "/usr/local/lib/python3.10/logging/__init__.py", line 1084, in flush
    self.stream.flush()
ValueError: I/O operation on closed file.

salt/_logging/handlers.py

@@ -65,7 +65,11 @@ def flush(self):
                 super().handle(record)
             finally:
                 self.__emitting = False
-        super().flush()
+        # Seeing an exception from calling flush on a closed file in the test
+        # suite. Handling this condition for now but this seems to be
+        # indicitive of an un-clean teardown at some point.
+        if not self.stream.closed:
+            super().flush()
 
     def sync_with_handlers(self, handlers=()):
         """