Mocking Guzzle to handle client requests in a test

KernelTestWithMockedClientRequestsExampleTest.php

<?php

namespace Drupal\Tests\external_entities\Kernel;

use Drupal\Core\DependencyInjection\ContainerBuilder;
use Drupal\KernelTests\KernelTestBase;
use GuzzleHttp\Promise\FulfilledPromise;
use GuzzleHttp\Psr7\Response;
use Psr\Http\Message\RequestInterface;

/**
 * Exanple of mocking a Guzzle response in a test class.
 */
class KernelTestWithMockedClientRequestsExampleTest extends KernelTestBase {

  /**
   * The modules to enable.
   *
   * @var array
   */
  protected static $modules = [
    'system',
    'user',
  ];

  /**
   * {@inheritdoc}
   */
  public function register(ContainerBuilder $container) {
    parent::register($container);

    // Register this test class as an HTTP client middleware.
    $container
      ->register('mocked_external_entity_source', static::class)
      ->addTag('http_client_middleware');
  }

  /**
   * Guzzle client middleware callback.
   */
  public function __invoke() {
    return function (callable $handler) {
      return function (RequestInterface $request, array $options) use ($handler) {
        $json = json_encode([
          'some data',
        ]);
        $response = new Response(200, [], $json);
        return new FulfilledPromise($response);
      };
    };
  }

  /**
   * Test method.
   */
  public function testExternalRequest(): void {
    $client = $this->container->get('http_client');
    $response = $client->request('GET', 'http://example.com');
    $body = $response->getBody()->getContents();
    dump($body);
  }

}