Change result note key from a string to a list of strings

[happz]

This better represents its real nature: a collection of various notes, there may be more than one note or topic tmt needed to share with user depending on what happened while the test was running.

Pull Request Checklist

• [x] implement the feature
• [ ] write the documentation
• [ ] extend the test coverage
• [x] update the specification
• [x] modify the json schema
• [ ] mention the version
• [ ] include a release note