A bizarre mock that works

test.php

<?php

namespace Tests;

use GuzzleHttp\Client;
use Tests\TestCase;

/**
 * Any Test
 *
 * @package Tests
 */
class AnyTest extends TestCase
{
    /**
     * @return void
     */
    public fuction testPleaseINeedToBeHappy(): void
    {
        $client = self::getHttpClientMock();

        $response = $client->post('', [
            'json' => [
                'query' => '...',
                'variables' => [
                    'foo' => 'bar',
                ],
            ],
        ]);

        $contentData = $response
            ->getBody()
            ->getContents()
        ;

        /* ... */
    }

    /**
     * @return Client
     */
    public static function getHttpClientMock(): Client
    {
        $client = \Mockery::spy(Client::class);

        $client
            ->shouldReceive('post')
            ->with(
                '',
                [
                    'json' => [
                        'query' => '...',
                        'variables' => [
                            'foo' => 'bar',
                        ],
                    ],
                ]
            )
            ->andReturn(self::getResponseMock())
        ;

        return $client;
    }

    /**
     * @return object
     */
    public static function getResponseMock()
    {
        return new class
        {
            /**
             * @return object
             */
            public function getBody()
            {
                return new class
                {
                    /**
                     * @return string
                     */
                    public function getContents(): string
                    {
                        return \json_encode([
                            'baz' => 'qux',
                        ]);
                    }
                };
            }
        };
    }
}