fix(admin-vite-plugin): Normalize file paths and add tests (#9595)

**What**
- #9338 had a regression which caused the import path in some virtual modules to be invalid on Windows.
- This PR fixes the issue so we now again create the correct import paths, and adds tests to prevent this from slipping in again.

packages/admin/admin-vite-plugin/vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from "vitest/config"
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: "node",
+    include: ["**/*.spec.{js,mjs,cjs,ts,mts,cts,jsx,tsx}"],
+  },
+})