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.
This allows the ability to provide arguments to a function, such as
`--on-event` in order to trigger a function on the
`fish_command_not_found` event, for example.
PR #1063
- If a function is defined, check that the function file exists and
that the contents matches a given string.
- If no functions exists, the functions folder should not exist.
- Verify plugin functionality.
