Fix Storage integration test (#13258)

Author: paulb777

FirebaseStorage/Tests/Integration/StorageIntegration.swift

@@ -204,7 +204,8 @@ class StorageResultTests: StorageIntegrationCommon {
         case let .unauthorized(bucket, object, serverError):
           XCTAssertEqual(bucket, "ios-opensource-samples.appspot.com")
           XCTAssertEqual(object, file)
-          XCTAssertNil(serverError)
+          XCTAssertNotNil(serverError)
+          XCTAssertEqual(serverError["ResponseErrorCode"] as? Int, 403)
           expectation.fulfill()
         default:
           XCTFail("Failed with unexpected error: \(error)")