qa: Close stdout and stderr file when node stops

test/functional/test_framework/test_node.py

@@ -200,6 +200,9 @@ class TestNode():
         if stderr != expected_stderr:
             raise AssertionError("Unexpected stderr {} != {}".format(stderr, expected_stderr))
 
+        self.stdout.close()
+        self.stderr.close()
+
         del self.p2ps[:]
 
     def is_node_stopped(self):