Use CocoaPods-based RNTesterPods for iOS tests (#25416)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/25416

Use CocoaPods-based RNTesterPods workspace to run iOS unit tests and integration tests.

This is necessary as new iOS projects now use CocoaPods by default. CocoaPods also powers the new package auto-linking feature.

In order to provide test coverage for this new default configuration, our iOS tests are being migrated to use a CocoaPods-managed RNTester workspace. This applies to both Circle CI, and Sandcastle.

Changelog:

[iOS] [Changed] - Use RNTesterPods for iOS Tests

Reviewed By: fkgozali

Differential Revision: D16052466

fbshipit-source-id: 724b0c51008882d3c06a9074693fe23e74abe86b