Add a test with a profile with a config/sync directory

A profile can contain a config/sync directory with a core.extension.yml to allow it to be installed from configuration. With Config Overlay, that allows having a completely empty config sync directory.

We should add a test for that.

Core's \InstallerExistingConfigTestBase does not allow for this scenario, because the creation of the profile's config/sync is coupled to also using that directory as the config sync directory. Not sure why that is, but I don't think it necessarily needs to be that way so I think we should open an issue to either remove that coupling or to at least make it configurable.