Add `cabal-testsuite` as package for changelogs

**Describe the feature request**
Can we produce changelogs for `cabal-testsuite`? It is an important part of the project. This would be for changes in these modules:

```pre
$ tree cabal-testsuite/src/Test
cabal-testsuite/src/Test
└── Cabal
├── CheckArMetadata.hs
├── DecodeShowBuildInfo.hs
├── Monad.hs
├── NeedleHaystack.hs
├── OutputNormalizer.hs
├── Plan.hs
├── Prelude.hs
├── Run.hs
├── Script.hs
├── Server.hs
├── TestCode.hs
└── Workdir.hs
```

I tried doing that and got this error:

```yaml
---
synopsis: Something changed with the test suite?
packages: [cabal-testsuite]
---
```

https://github.com/haskell/cabal/actions/runs/28534834398/job/84593427631?pr=12069

```
Validation error(s) in entry `changelog.d/12069.md`:
Unknown package cabal-testsuite
error: Validation failed.
```

Start with the failing changelog entry in changelog.d/12069.md and the validation error from the linked GitHub Actions run, then inspect how package names are registered for changelogs. Confirm that cabal-testsuite is accepted for changes under cabal-testsuite/src/Test/Cabal, and rerun the changelog validation to verify the entry passes.

Written by the indexing model from the issue text.