test: use async import pattern in skip-on-lint snap test

The heavy-plugin.ts throws on import, proving the dynamic import
inside vitePlugins() is never executed during vp lint.

Author: fengmk2

packages/cli/snap-tests/vite-plugins-skip-on-lint/heavy-plugin.ts

@@ -0,0 +1,5 @@
+throw new Error('Plugins should not be loaded during lint');
+
+export default function heavyPlugin() {
+  return { name: 'heavy-plugin' };
+}

packages/cli/snap-tests/vite-plugins-skip-on-lint/vite.config.ts

@@ -2,8 +2,9 @@ import { defineConfig, vitePlugins } from 'vite-plus';
 
 export default defineConfig({
   plugins: [
-    vitePlugins(() => {
-      throw new Error('Plugins should not be loaded during lint');
+    vitePlugins(async () => {
+      const { default: heavyPlugin } = await import('./heavy-plugin');
+      return [heavyPlugin()];
     }),
   ],
 });