Add AppCheck to SwiftPM tests (#8456)

Author: ncooke3

SwiftPMTests/swift-test/main.swift

@@ -17,6 +17,7 @@ import Firebase
 import FirebaseCore
 import FirebaseAuth
 import FirebaseABTesting
+import FirebaseAppCheck
 #if os(iOS) && !targetEnvironment(macCatalyst)
   import FirebaseAppDistribution
 #endif