Include `.gradle-test-kit` in caches as well

I'm not seeing any logic in place for caching the directory generated by testkit. It might be nice if that were done automatically as well.

We currently have this logic in `ktlint-gradle` to do this:

```yml
- name: Restore TestKit cache
# Inspired by https://github.com/actions/cache/issues/432#issuecomment-740376179
uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4.2.3
with:
path: |
plugin/.gradle-test-kit/caches/modules-2
plugin/.gradle-test-kit/caches/files-2.1
plugin/.gradle-test-kit/caches/metadata-2.96
key: ${{ runner.os }}-gradle-wrapper-${{ hashFiles('**/gradlew') }}-${{ github.sha }}
restore-keys: |
${{ runner.os }}-gradle-wrapper-${{ hashFiles('**/gradlew') }}-
${{ runner.os }}-gradle-wrapper-
```

Start by locating the existing cache configuration for the Gradle build and compare it with the `ktlint-gradle` example in the issue. Add caching for `plugin/.gradle-test-kit/caches/modules-2`, `files-2.1`, and `metadata-2.96`; done means TestKit dependencies are restored automatically by the workflow.

Written by the indexing model from the issue text.