Clean up objc-test-ios.sh, include tvOS build check

Summary:
**Motivation**: Having full tests for both iOS and tvOS is hard on Travis, and it's probably ok to not run full tests on tvOS.  However, we should make sure that tvOS at least builds and doesn't have missing files.

Also fixed a warning import.
Closes https://github.com/facebook/react-native/pull/12642

Differential Revision: D4661571

Pulled By: mkonicek

fbshipit-source-id: 45932113951c01e35d6e8ce91af2522cb135efe8