add yaml linting of manifest.yml to unit tests

There is a possibility that manual editing of the manifest can result in indent errors, which will stop a deploy.

Use https://www.npmjs.com/package/yaml-lint

Start by inspecting the unit-test setup and manifest.yml, then read the yaml-lint package documentation linked in the issue. Add a test that lints manifest.yml and ensure the test suite reports indentation errors before deployment.

Written by the indexing model from the issue text.