Change Auth.backend property to constant

Author: ncooke3

FirebaseAuth/Sources/Swift/Auth/Auth.swift

@@ -2337,7 +2337,7 @@ extension Auth: AuthInterop {
   ///   Auth's backend.
   var requestConfiguration: AuthRequestConfiguration
 
-  var backend: AuthBackend
+  let backend: AuthBackend
 
   #if os(iOS)
 

FirebaseAuth/Tests/Unit/OAuthProviderTests.swift

@@ -282,7 +282,10 @@ import FirebaseCore
         .replacingOccurrences(of: ")", with: "")
       FirebaseApp.configure(name: strippedName, options: options)
       OAuthProviderTests.auth = Auth.auth(app: FirebaseApp.app(name: strippedName)!)
-      OAuthProviderTests.auth?.backend = authBackend
+      OAuthProviderTests.auth = Auth(
+        app: FirebaseApp.app(name: strippedName)!,
+        backend: authBackend
+      )
       OAuthProviderTests.auth?.mainBundleUrlTypes =
         [["CFBundleURLSchemes": [scheme]]]
     }