Add onnx / test to required merge rules (#78790)
Add two `linux-xenial-py3.7-clang7-onnx / test` checks to required merge rules for `torch.onnx`
~~Question: Do I need to full name (`linux-xenial-py3.7-clang7-onnx / test (default, 1, 2, linux.2xlarge)`) or will the bot match the prefix?~~
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78790
Approved by: https://github.com/BowenBao, https://github.com/janeyx99
