chore: restores removed tests (#39452)
chore: restores removed tests (#39452)

### 📖 What's in there?

As part of the 4 hands work done in #39397, we accidentally removed a describe for middleware "using a single matcher with i18n" (and no base path nor trailing slash).

This PR restores it.

### 🧪 How to test?

```
pnpm testheadless --testPathPattern middleware-matcher
```

### 🆙 Note to reviewers

There's only a couple lines with meaningful change in this PR. Hide whitespaces in github's diff screen to find it more easily: https://github.com/vercel/next.js/pull/39452/files?diff=split&w=1
JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js