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

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

[happz]

@psss @thrix @lukaszachy This is definitely a breaking change for TF, and potentially for other users as well. TF ingests the field and treats it as a string, it will require at least two patches to accommodate the change - simple ones, but still.

Yet I believe this change should happen because it better reflects the nature of the note field: there can be multiple notes attached to a single test, and 1. code needs to take extra measures to stack notes correctly, and 2. this robs consumer from processing them, e.g. filtering or rendering accordingly to their meaning for the end user. For example, a test with a failed test check that failed to reboot its guest & enforces interpretation of its result by setting its result field may collect three notes - and such a scenario is not that unusual, reboots are common, outages too, and checks are getting more and more traction. By using a list, we would make the code simpler, it'd let consumers format and mangle them as they need, and it'd be easier to add new notes - which I think will be a considerable advantage WRT results of prepare and finish steps we don't generate yet, but I experimented a bit with those and some plugins may share extra details in their result notes, which was very tedious with note += ', foo...'.

So, how to proceed to make it as painless for the end user as possible? Do we know who out there consumes results.yaml?