Pin undici in tests (#73263)
undici 7 was released 30 minutes ago and broke the tests because
Turbopack now has troubles with its `node:sqlite` import.
You can see the test failure in
https://github.com/vercel/next.js/pull/73261#issuecomment-2504054040
```
Error: Cannot find module 'node:sqlite': Unsupported external type Url for commonjs reference
at <unknown> (.next/server/chunks/ssr/c1118_undici_e1ac85._.js:35:29147)
at [project]/node_modules/.pnpm/undici@7.0.0/node_modules/undici/lib/cache/sqlite-cache-store.js [app-rsc] (ecmascript) (.next/server/chunks/ssr/c1118_undici_e1ac85._.js:35:29278)
at instantiateModule (.next/server/chunks/ssr/[turbopack]_runtime.js:590:23)
at getOrInstantiateModuleFromParent (.next/server/chunks/ssr/[turbopack]_runtime.js:645:12)
at commonJsRequire (.next/server/chunks/ssr/[turbopack]_runtime.js:147:20)
at [project]/node_modules/.pnpm/undici@7.0.0/node_modules/undici/index.js [app-rsc] (ecmascript) (.next/server/chunks/ssr/c1118_undici_e1ac85._.js:116:5860)
at instantiateModule (.next/server/chunks/ssr/[turbopack]_runtime.js:590:23)
at getOrInstantiateModuleFromParent (.next/server/chunks/ssr/[turbopack]_runtime.js:645:12)
at esmImport (.next/server/chunks/ssr/[turbopack]_runtime.js:132:20)
at [project]/app/undici/page.js [app-rsc] (ecmascript) <internal part 2> (.next/server/chunks/ssr/[root of the server]__83c897._.js:1:5666) {
```
https://github.com/nodejs/undici/blob/1cfe0949053aac6267f11b919cee9315a27f1fd6/index.js#L52-L59
https://github.com/nodejs/undici/blob/1cfe0949053aac6267f11b919cee9315a27f1fd6/lib/cache/sqlite-cache-store.js#L3
