Delaying mock response for 5 seconds with GuzzleHttp - MockHandler

Question

I have the following PHP / PHPUnit code:

<?php

namespace Experiments\Tests;

use GuzzleHttp\Client as HttpClient;
use GuzzleHttp\Handler\MockHandler;
use GuzzleHttp\HandlerStack;
use GuzzleHttp\Psr7\Response;
use GuzzleHttp\RequestOptions;
use PHPUnit\Framework\TestCase;

final class ApiGatewayTest extends TestCase
{

    public function testExecute()
    {

        $req = json_decode('{ "root": { "param1": [ "value1" ] } }', true);
        $resp = '{ "echo": { "root": { "param1": [ "value1" ] } } }';

        $options = [
            RequestOptions::HEADERS => [
                'Content-Type' => 'application/json',
            ],
            RequestOptions::JSON =>  $req,
            RequestOptions::TIMEOUT => 10,
        ];

        $handlerStack = HandlerStack::create(
            new MockHandler([new Response(200, ['Content-Type' => 'application/json'], $resp)])
        );

        $client = new HttpClient(['handler' => $handlerStack]);

        $response = $client->request(
            'POST',
            'whatever-just-mocking',
            $options
        );

        $responseBodyJson = (string) $response->getBody();
        $responseBody = json_decode($responseBodyJson, true);

        $this->assertSame('value1', $responseBody['echo']['root']['param1'][0]);
    }
}

The unit test above works just fine. You can test it with:

$ ./vendor/bin/phpunit --filter 'testExecute\b'

What I need: Delay the mock response for 5 seconds, so I can implement and test a timeout handling in the same class above.

Any idea on how to do that?

Thanks!