tests: linkcheck: InfiniteRedirectOnHeadHandler: close server-side co…

…nnection after response body is written, since we do not anticipate the client will read it

Ref: sphinx-doc#11340 (comment)

tests/test_build_linkcheck.py

@@ -521,6 +521,7 @@ def do_GET(self):
         self.send_header("Content-Length", str(len(content)))
         self.end_headers()
         self.wfile.write(content)
+        self.close_connection = True  # we don't expect the client to read this response body
 
 
 @pytest.mark.sphinx('linkcheck', testroot='linkcheck-localserver', freshenv=True)