add an unit test target to TestApp (#29962)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29962
### Summary
Recently we've found that the master branch was constantly broken due to some unwanted change being landed on mobile. The problem is that our CI was not able to detect the runtime errors. Starting from this PR, we'll add some unit tests to the iOS Simulator build. As follows:
1. Add an unit test target to XCode (this PR)
2. Use Fastlane to run the tests on CI
3. Modify the CI scripts to trigger tests
### Test Plan
- Don't break the existing CI jobs unless they are flaky.
Test Plan: Imported from OSS
Differential Revision: D18582908
Pulled By: xta0
fbshipit-source-id: f960c47d3bbda79e754a0513e8711867fd3588d2
