[ONNX] Create common ExportTestCase base class (#88145)
Refactor out a common base class `ExportTestCase`, for common things in `setUp`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88145
Approved by: https://github.com/justinchuby, https://github.com/abock, https://github.com/AllenTiTaiWang
