use waitFor in place of act (#201921)

## Summary

Fixes incorrect usage of act for assertions. Informed from
https://buildkite.com/elastic/kibana-pull-request/builds/254649#019369dd-02e4-4d1e-9c45-face1da80884

### Before

<img width="1239" alt="Screenshot 2024-11-27 at 10 46 23"
src="https://github.com/user-attachments/assets/89fb0142-add8-401d-b270-d9bc8a3cbe23">

## After

<img width="1052" alt="Screenshot 2024-11-27 at 10 46 34"
src="https://github.com/user-attachments/assets/61f4d278-676c-4761-a003-1e22714cf5b7">


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x-pack/plugins/fleet/public/components/agentless_enrollment_flyout/index.test.tsx

@@ -5,7 +5,7 @@
  * 2.0.
  */
 import React from 'react';
-import { act, waitFor } from '@testing-library/react';
+import { waitFor } from '@testing-library/react';
 
 import { sendGetAgents, useGetPackageInfoByKeyQuery } from '../../hooks';
 import { usePollingIncomingData } from '../agent_enrollment_flyout/use_get_agent_incoming_data';
@@ -64,7 +64,7 @@ describe('AgentlessEnrollmentFlyout', () => {
     const { getByText } = renderer.render(
       <AgentlessEnrollmentFlyout onClose={onClose} packagePolicy={packagePolicy} />
     );
-    await act(async () => {
+    await waitFor(async () => {
       expect(getByText('Confirm agentless enrollment')).toBeInTheDocument();
       expect(getByText('Step 1 is loading')).toBeInTheDocument();
       expect(