Enable the unicorn/prefer-at ESLint plugin rule (PR 15008 follow-up)

Please find additional information here:
 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at
 - https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-at.md

test/resources/reftest-analyzer.js

@@ -229,7 +229,7 @@ window.onload = function () {
         /^ {2}IMAGE[^:]*\((\d+\.?\d*)x(\d+\.?\d*)x(\d+\.?\d*)\): (.*)$/
       );
       if (match) {
-        const item = gTestItems[gTestItems.length - 1];
+        const item = gTestItems.at(-1);
         item.images.push({
           width: parseFloat(match[1]),
           height: parseFloat(match[2]),

test/test.js

@@ -13,7 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* eslint-disable no-var */
+/* eslint-disable no-var, unicorn/prefer-at */
 
 "use strict";
 

test/unit/ui_utils_spec.js

@@ -301,7 +301,7 @@ describe("ui_utils", function () {
           ids.add(view.id);
         }
       }
-      return { first: views[0], last: views[views.length - 1], views, ids };
+      return { first: views[0], last: views.at(-1), views, ids };
     }
 
     // This function takes a fixed layout of pages and compares the system under