firebase-ios-sdk

Add remaining tests to Xcode (#1256)

* Add classes in testutil and util to the project

  * Exclude util/iterator_adaptors_test which requires gmock
  * Exclude remote/serializer_test which is missing some paths

* Add iterator_adaptors_test and add gmock support

  * Add gmock support to the GoogleTest podspec we vendor
  * Add iterator_adaptors_test.cc to the Xcode project

* Add a script that verifies all tests are referenced in the project

* Add a travis check that all tests are referenced in the project

* Review feedback

* Moar feedback