Tear down module logger handler in system tests

The module-level logger has a handler that writes into a temporary
directory. Ensure the logging output is flushed and the handler is
closed before attempting to remove this temporary directory.

bin/tests/system/conftest.py

@@ -449,6 +449,8 @@ else:
                 )
             else:
                 mlogger.debug("deleting temporary directory")
+                handler.flush()
+                handler.close()
                 shutil.rmtree(testdir)
 
     def _run_script(  # pylint: disable=too-many-arguments