Add iOS test app skeleton (#26261)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26261
### Summary
Previously we have enabled the CI jobs for Pull Requests and nightly build
- **#25840 [iOS][Circle CI] Add PR jobs for iOS builds**
- **#26074 [IOS][CIRCLE CI] Nightly jobs for iOS builds**
The testing phase is missing in the nightly build process. Although we are able to generate the build and upload it to the AWS, there is no way to know whether the binary is valid or not (there could be a linking error). To add the test phase to the process, we need
1. Put a dummy test App in the repo.
2. After the build jobs finishes, manually link the static libs to the dummy app to produce an executable using the xcode tool chain.
3. If there is no linking error, then upload the binaris to AWS. If there is an error, then stops the following process and reports an error in CI.
The second and third steps depends on the first step which needs to be landed first.
### Test Plan
- Don't break any existing CI jobs
Test Plan: Imported from OSS
Differential Revision: D17408929
Pulled By: xta0
fbshipit-source-id: e391da242639943005453d1318795f981034cc72
