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

tests/browser/scoped-custom-element-registry.ts

@@ -0,0 +1,11 @@
+// The card renders a few elements that give their shadow root a registry of its
+// own (`ScopedRegistryHost` from `@lit-labs/scoped-registry-mixin`, used by the
+// media filter and the select). That is not something a browser can do
+// natively: it needs this polyfill, which Home Assistant loads for the whole
+// frontend. Without it, rendering one of those elements throws "importNode is
+// not a function" as Lit tries to clone a template into a registry that has no
+// document behind it.
+//
+// It replaces `customElements` and `attachShadow`, so it has to be the first
+// thing the page runs -- hence its own setup file, ahead of every other.
+import '@webcomponents/scoped-custom-element-registry/scoped-custom-element-registry.min';