Add test to cover the fields filter option

tests/fixtures/fields-expected-items.json

+{
+  "data": [
+    {
+      "type": "menu_link_content--menu_link_content",
+      "id": "jsonapi_menu_test.open",
+      "attributes": {
+        "title": "Home",
+        "url": "%base_pathmenu_name_test"
+      }
+    },
+    {
+      "type": "menu_link_content--menu_link_content",
+      "id": "menu_link_content:%uuid",
+      "attributes": {
+        "title": "%title",
+        "url": "%base_pathmenu_callback_title"
+      }
+    },
+    {
+      "type": "menu_link_content--menu_link_content",
+      "id": "jsonapi_menu_test.user.login",
+      "attributes": {
+        "title": "Login",
+        "url": "%base_pathuser\/login"
+      }
+    }
+  ]
+}

tests/src/Functional/JsonapiMenuItemsTest.php

@@ -126,6 +126,30 @@ class JsonapiMenuItemsTest extends BrowserTestBase {
     self::assertCacheContext($headers, 'url.query_args:filter');
   }
 
+  /**
+   * Tests the JSON:API Menu Items resource with the 'fields' filter.
+   */
+  public function testParametersFields() {
+    $link_title = $this->randomMachineName();
+    $content_link = $this->createMenuLink($link_title, 'jsonapi_menu_test.open');
+
+    $url = Url::fromRoute('jsonapi_menu_items.menu', [
+      'menu' => 'jsonapi-menu-items-test',
+      'filter' => [
+        'fields' => "title,url",
+      ],
+    ]);
+    [$content] = $this->getJsonApiMenuItemsResponse($url);
+    $this->assertCount(3, $content['data']);
+
+    $expected_items = Json::decode(strtr(file_get_contents(dirname(__DIR__, 2) . '/fixtures/fields-expected-items.json'), [
+      '%uuid' => $content_link->uuid(),
+      '%title' => $link_title,
+      '%base_path' => Url::fromRoute('<front>')->toString(),
+    ]));
+    $this->assertEquals($expected_items['data'], $content['data']);
+  }
+
   /**
    * Tests the JSON:API Menu Items resource with the 'parents' filter.
    */