chore(tests): linux error handling to be behind flags as in pytorch

https://github.com/facebookresearch/xformers/blob/ca6d2aa0d43241fc8a8dcd872debc2406160160d/tests/multiprocessing_utils.py#L70

https://github.com/pytorch/pytorch/blob/0f1785d45c0a7157e86abc214e1ee0684ef8d4a6/torch/testing/_internal/common_distributed.py#L960-L961

needed for https://github.com/NixOS/nixpkgs/pull/513735