chore(aws-cognito): fix broken integration tests (#19684)

----

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### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
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packages/@aws-cdk/aws-cognito/test/integ.user-pool-client-explicit-props.expected.json

@@ -20,6 +20,16 @@
         },
         "EmailVerificationMessage": "The verification code to your new account is {####}",
         "EmailVerificationSubject": "Verify your new account",
+        "Schema": [
+          {
+            "AttributeDataType": "String",
+            "Name": "attribute_one"
+          },
+          {
+            "AttributeDataType": "String",
+            "Name": "attribute_two"
+          }
+        ],
         "SmsVerificationMessage": "The verification code to your new account is {####}",
         "VerificationMessageTemplate": {
           "DefaultEmailOption": "CONFIRM_WITH_CODE",

packages/@aws-cdk/aws-cognito/test/integ.user-pool-client-explicit-props.ts

@@ -1,11 +1,15 @@
 import { App, RemovalPolicy, Stack } from '@aws-cdk/core';
-import { OAuthScope, UserPool, ClientAttributes } from '../lib';
+import { OAuthScope, UserPool, ClientAttributes, StringAttribute } from '../lib';
 
 const app = new App();
 const stack = new Stack(app, 'integ-user-pool-client-explicit-props');
 
 const userpool = new UserPool(stack, 'myuserpool', {
   removalPolicy: RemovalPolicy.DESTROY,
+  customAttributes: {
+    attribute_one: new StringAttribute(),
+    attribute_two: new StringAttribute(),
+  },
 });
 
 userpool.addClient('myuserpoolclient', {