Add a test to cover promises

tests/GraphQLTest.php

@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace GraphQL\Tests;
+
+use GraphQL\Executor\Promise\Adapter\SyncPromiseAdapter;
+use GraphQL\GraphQL;
+use GraphQL\Type\Definition\ObjectType;
+use GraphQL\Type\Definition\Type;
+use GraphQL\Type\Schema;
+use PHPUnit\Framework\TestCase;
+use function sprintf;
+
+class GraphQLTest extends TestCase
+{
+    public function testPromiseToExecute() : void
+    {
+        $promiseAdapter = new SyncPromiseAdapter();
+        $schema         = new Schema(
+            [
+                'query' => new ObjectType(
+                    [
+                        'name' => 'Query',
+                        'fields' => [
+                            'sayHi' => [
+                                'type' => Type::nonNull(Type::string()),
+                                'args' => [
+                                    'name' => [
+                                        'type' => Type::nonNull(Type::string()),
+                                    ],
+                                ],
+                                'resolve' => static function ($value, $args) use ($promiseAdapter) {
+                                    return $promiseAdapter->createFulfilled(sprintf('Hi %s!', $args['name']));
+                                },
+                            ],
+                        ],
+                    ]
+                ),
+            ]
+        );
+
+        $promise = GraphQL::promiseToExecute($promiseAdapter, $schema, '{ sayHi(name: "John") }');
+        $result  = $promiseAdapter->wait($promise);
+        $this->assertSame(['data' => ['sayHi' => 'Hi John!']], $result->toArray());
+    }
+}