firebase-ios-sdk

Add support for Firestore unit tests on macOS (#2641)

* Rename Firestore/Example/App/macOS_example to macOS

* Update paths to macOS and names to Firestore_Example_macOS

* Add Firestore_Tests_macOS target

This uses a shared Tests Info.plist. This is important because it
configures NSPrincipalClass

* Teach sync_project about Firestore_Tests_macOS

* Add files to Firestore_Tests_macOS

* Add Firestore_Tests_macOS target to the Podfile

* Add HEADER_SEARCH_PATHS to match iOS

* Force C99/C++11 conformance

* pod install after adding Firestore_Tests_macOS

* Get macOS tests to link

* Add spec test json files to the macOS tests

* Remove development code signing

* Disable app sandboxing to allow GoogleTest death tests to succeed.