[export] fix test_passes on ci (#120322)
We put the test cases generation in unitest.setUp to avoid running export on machines that runs with Python 3.12, where dynamo is not supported.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120322
Approved by: https://github.com/angelayi, https://github.com/huydhn, https://github.com/malfet
