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.

scripts/browsers.ts

@@ -0,0 +1,27 @@
+const BROWSERS = ['chromium', 'firefox', 'webkit'] as const;
+
+export type Browser = (typeof BROWSERS)[number];
+
+const isValidBrowser = (name: string): name is Browser =>
+  BROWSERS.some((browser) => browser === name);
+
+/**
+ * Which browsers to run, all of them unless one is named. CI names one per job
+ * so that the three run at the same time on separate machines rather than one
+ * after another on one.
+ *
+ * Shared by every suite that mounts the card, so that naming a browser means
+ * the same thing to each of them.
+ */
+export const getBrowsers = (): readonly Browser[] => {
+  const requested = process.env.VITEST_BROWSER;
+  if (requested === undefined) {
+    return BROWSERS;
+  }
+
+  if (!isValidBrowser(requested)) {
+    throw new Error(`Unknown browser: ${requested}`);
+  }
+
+  return [requested];
+};