test: Add a test harness for the built card (#2653)

Mounts the card from `dist/` the way Home Assistant does, by URL with a
HACS tag, and asserts the entry stays a facade, that no chunk imports
it, that the browser is served the file unmodified, and that the card
starts up and fetches a language chunk lazily.

Runs in Chromium, Firefox and WebKit, so a change of bundler or minifier
is checked against every engine.

tests/browser/test-utils.ts

@@ -96,6 +96,9 @@ export interface StillCameraHASSOptions {
 
   // Anything else the card should be able to see, as entity ID to state.
   entities?: Record<string, FakeEntityOptions | string>;
+
+  // The language Home Assistant is set to for translation tests.
+  language?: string;
 }
 
 /**
@@ -111,6 +114,7 @@ export const createStillCameraHASS = (options?: StillCameraHASSOptions): FakeHAS
       ...options?.entities,
     },
     registry: Object.fromEntries(cameras.map((camera) => [camera, {}])),
+    ...(options?.language && { language: options.language }),
   });
 };