Fix open source C++ Firestore building with -DFIREBASE_IOS_BUILD_TESTS=OFF.
Fix open source C++ Firestore building with -DFIREBASE_IOS_BUILD_TESTS=OFF.

PiperOrigin-RevId: 316190122

firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk