test: fix flaky in customElements (#2109)

scripts/setup/lit.ts

@@ -0,0 +1,12 @@
+import { vi } from 'vitest';
+
+if (typeof window !== 'undefined') {
+  // Refs: https://github.com/jsdom/jsdom/blob/master/lib/jsdom/living/custom-elements/CustomElementRegistry-impl.js
+  const customElements = {
+    define: vi.fn(),
+    get: vi.fn(),
+    whenDefined: vi.fn(),
+    upgrade: vi.fn(),
+  };
+  vi.stubGlobal('customElements', customElements);
+}