This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
- Default value is set to static '$HOME/.zsh_history' -- dotDir is not
prepended anymore
- $HOME is not prepended to the option value
- Ensure history path directory exists
Fixes#886, replaces #427.