Caching suggestions for planemo test on TravisCI

Hello all. Are there any best practises for the TravisCI cache settings when using planemo test? I know that the IUC tools have moved away from TravisCI to GitHub Actions, so perhaps this never came up.

In https://github.com/peterjc/galaxy_blast/commit/91b311a12ac8696b5eee3628176edb572806c1c7 I tried the following but saw little benefit (however I am currently using a local checkout of Galaxy outside of planemo's control):

```
cache:
- pip
- $HOME/.planemo
```

Are there subfolders within that which are better cached?