test: Add browser tests for the media viewing gallery and viewer (#2661)

vitest.browser.config.ts

@@ -67,13 +67,23 @@ export default defineConfig({
     name: 'browser',
     include: ['tests/**/*.browser.test.ts'],
 
+    // One file at a time. Files otherwise share a browser, in which only one of
+    // them can hold focus, so a test driving the keyboard loses it to whichever
+    // sibling clicks next. Measured no slower than running in parallel.
+    fileParallelism: false,
+
     // The tests under `tests/dist` need a card build to exist (in dist/), which
     // this suite does not require. They have their own config, see
     // vitest.dist.config.ts .
     exclude: ['tests/dist/**'],
 
-    // Cosmetic: Style the pages as HA does for screenshots.
-    setupFiles: ['./tests/browser/style.ts'],
+    setupFiles: [
+      // Must run before anything defines an element.
+      './tests/browser/scoped-custom-element-registry.ts',
+
+      // Cosmetic: Style the pages as HA does for screenshots.
+      './tests/browser/style.ts',
+    ],
 
     // A failing test leaves a screenshot and any attachments behind. Both
     // default to somewhere else -- a `__screenshots__` directory next to the
@@ -85,6 +95,11 @@ export default defineConfig({
     // past the default of 300ms.
     slowTestThreshold: 10000,
 
+    // How long a test may take before it is called a hang. Mounting a real card
+    // in three browsers at once is expensive, and a machine running something
+    // else alongside can push a healthy test past the default of 5 seconds.
+    testTimeout: 30000,
+
     server: {
       deps: {
         // These dependencies import without extensions.