[ONNX] Clean up unit tests, rename files and improve import style (#81141)
- Rename `test_pytorch_common` -> `pytorch_test_common`, `test_onnx_common` -> `onnx_test_common`, removing the test_ prefix to show that the files are not test cases
- Remove import * in `test_pytorch_common` and adjust to import from `testing._internal.common_utils` (where functions are actually defined) instead
- Import modules only in `test_pytorch_onnx_onnxruntime` (too many to handle in a single PR in other tests) (The skips are exceptions)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81141
Approved by: https://github.com/BowenBao
