[Firestore] Fix build-for-testing linking affecting CocoaPods (Approach 1) (#11977)

Author: ncooke3

FirebaseFirestore.podspec

@@ -19,6 +19,8 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
 
   s.swift_version = '5.3'
 
+  s.weak_framework = 'FirebaseFirestoreInternal'
+
   s.cocoapods_version = '>= 1.4.0'
   s.prefix_header_file = false