Add test for the unsupported browser screen (#12787)

* Add test for the unsupported browser screen

Test a handful of APIs that are easy to disable with an init script
and assert that the app display the "Unsupported Browser" screen.

* Oops, test wasn't actually testing anything

* Remove other feature tests

They didn't work because there are two separate screens, one of
which has an iframe and one of which doesn't.

* Add screenshot test

* Rename to ts

* Fix screenshot

* Fix snapshot filename

* Rename file

* rename screenshot again

* rename screenshot again

* 16th time's a charm

* remove file

* please just be in the right place

playwright/e2e/app-loading/feature-detection.spec.ts

@@ -0,0 +1,27 @@
+/*
+Copyright 2024 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { test, expect } from "../../element-web-test";
+
+test(`shows error page if browser lacks Intl support`, async ({ page }) => {
+    await page.addInitScript({ content: `delete window.Intl;` });
+    await page.goto("/");
+
+    const header = await page.frameLocator("iframe").getByText("Unsupported browser");
+    await expect(header).toBeVisible();
+
+    await expect(page).toMatchScreenshot("unsupported-browser.png");
+});