test: run frontend unit tests in browsers (#38860)

Run them in headless [vitest browser
mode](https://vitest.dev/guide/browser/) in chromium and firefox.
Similar UX than current tests, it's about 5 times as slow (goes from 1s
to 5s on my machine), but definitely worth it as it removes all
happy-dom problems.

---------

Signed-off-by: silverwind <me@silverwind.io>

web_src/js/utils/testhelper.ts

@@ -1,8 +1,15 @@
-// there could be different "testing" concepts, for example: backend's "setting.IsInTesting"
-// even if backend is in testing mode, frontend could be complied in production mode
-// so this function only checks if the frontend is in unit testing mode (usually from *.test.ts files)
-export function isInFrontendUnitTest() {
-  return import.meta.env.MODE === 'test';
+import {onTestFinished} from 'vitest';
+
+/** Record and block navigations, as a real browser forbids stubbing "window.location" */
+export function captureNavigations() {
+  const navigations: Array<{url: string, type: NavigationType}> = [];
+  const onNavigate = (e: NavigateEvent) => {
+    navigations.push({url: e.destination.url, type: e.navigationType});
+    e.preventDefault();
+  };
+  window.navigation.addEventListener('navigate', onNavigate);
+  onTestFinished(() => window.navigation.removeEventListener('navigate', onNavigate));
+  return navigations;
 }
 
 /** strip common indentation from a string and trim it */