test: load ICU data once before Vitest workers start

tests/vitest-global-setup.ts

@@ -0,0 +1,7 @@
+// The first date format on a machine pages Node's ICU data in from disk, and
+// every later process shares those pages. On Windows CI that read can stall a
+// test past its timeout while the first wave of workers loads, so pay it here,
+// once, before any worker starts.
+export default function warmIcuData(): void {
+	new Date(0).toLocaleDateString();
+}

vitest.config.mts

@@ -46,6 +46,7 @@ export default defineConfig({
 		// Files that need a DOM or Svelte code opt in with `// @vitest-environment jsdom`.
 		// jsdom setup per file is the suite's largest cost, so it is not the default.
 		setupFiles: ["./tests/vitest-setup.ts"],
+		globalSetup: ["./tests/vitest-global-setup.ts"],
 		coverage: {
 			provider: "v8",
 			include: ["src/**/*.ts"],