test: default vitest root projects to threads

vitest.ui.config.ts

@@ -3,8 +3,12 @@ import { createScopedVitestConfig } from "./vitest.scoped-config.ts";
 export function createUiVitestConfig(env?: Record<string, string | undefined>) {
   return createScopedVitestConfig(["ui/src/ui/**/*.test.ts"], {
     dir: "ui/src/ui",
+    environment: "jsdom",
     env,
+    isolate: true,
     name: "ui",
+    setupFiles: ["ui/src/test-helpers/lit-warnings.setup.ts"],
+    useNonIsolatedRunner: false,
   });
 }