add unit tests to iOS CI jobs (#30133)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30133
### 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.
### Previous work
- Add an unit test target to the iOS TestApp ( #29962 )
- Update Fastlane to run tests ( #29963 )
### What's been changed in CI
1. XCode version has been updated to 11.2.1
2. For iOS simulator build, we'll run some unit tests( currently only one) after the build test.
Test Plan: Imported from OSS
Differential Revision: D18641413
Pulled By: xta0
fbshipit-source-id: 12942206f1dee045b2addba3ae618760e992752c
