Import Firebase Swift APIs into FirebasePodTest source. (#7756)

Author: ncooke3

CoreOnly/Tests/FirebasePodTest/FirebasePodTest/AppDelegate.swift

@@ -14,6 +14,11 @@
 
 import UIKit
 import Firebase
+// Verify that the following Firebase Swift APIs can be found.
+import FirebaseAnalyticsSwift
+import FirebaseFirestoreSwift
+import FirebaseInAppMessagingSwift
+import FirebaseStorageSwift
 
 class CoreExists: FirebaseApp {}
 class AnalyticsExists: Analytics {}