Refactors test to not require fixture config.yml syncing.
Book layouts can use yield_for_subnav to render properly named subnav partials [#64252280]

- A proper name would be the same as the root folder of the 'directory'
  supplied for a particular document repo in config.yml.
  For example:
  - github_repo: some-org/some-repo
    directory: some-folder/some-subfolder

    For this repo, the properly named subnav partial would be:
    subnavs/_some-folder.erb
Silence I18n warnings
Config.yml specifies arbitrarily named subnav_templates. [finishes #64252280]

Defaults to 'default.erb'
Bump version to 0.0.8 for subnav_template feature
Fixtures include a missing subdirectory.