Use expecttest from PyPI (#60658)
Summary:
This PR removes `torch/testing/_internal/expecttest.py` in favor of https://github.com/ezyang/expecttest. See also https://github.com/ezyang/ghstack/pull/71.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60658
Test Plan: CI.
Reviewed By: ezyang
Differential Revision: D29430763
Pulled By: samestep
fbshipit-source-id: b7cdc7ba37330176149fd465312118e2254ae92e
