[tensorexpr] Switch cpp tests to pure gtest (#48160)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48160
We no longer use the custom c++ test infra anyways, so move to pure
gtest.
Fixes #45703
ghstack-source-id: 116977283
Test Plan: `buck test //caffe2/test/cpp/tensorexpr`
Reviewed By: navahgar, nickgg
Differential Revision: D25046618
fbshipit-source-id: da34183d87465f410379048148c28e1623618553
