Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39277

When we were reworking the Build logic in CI, we thought that the `test_ios` job and the `test_ios_rntester` jobs were the same. But, actually, they aren't.
So, now, we are seeing failures in the `test_ios` because its caches are not updated.

This changes remove the duplication of the two jobs, making sure that they use the same job, adding the possibility to run tests directly in rntester.

## Changelog:
[Internal] - Reuse test_ios_rntester to run iOS tests

Reviewed By: motiz88

Differential Revision: D48950096

fbshipit-source-id: e297d77fbd18f03873ed7a16a595e186d0e2453a