disable Onnx test for google/long-t5-tglobal-base
# What does this PR do?

For `("longt5", "google/long-t5-tglobal-base")`, we get

```bash
Floating point exception (core dumped)
```
in this call
https://github.com/huggingface/transformers/blob/fc546332d7a9395323f656635362c9e0f3c4161a/src/transformers/onnx/convert.py#L404

Let's disable it for now, so other Onnx tests could be run.

[Failed job run](https://github.com/huggingface/transformers/runs/6892306185?check_suite_focus=true)
Adding @regisss as a reviewer as this is suggested by GitHub automatically 😄 <|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>I will tag lewis when he is back.<|||||>Hi @lewtun !

Could you take a look at this ONNX test? Thank you.