Add tests for Base and OverlayLayerDirective

[atd-schubert]

…also add them to the index and fix a bug when reading the caption value.

Please fill in this template.

• [x] Use a meaningful title for the pull request.
• [x] Test the change in your own code. (Run with npm test.)
• [x] Add or edit tests to reflect the change.
• [x] Fulfill linting rules. (Run with npm run lint.)
• [ ] Add inline documentation.

Closes #423