Add test for basic runtime validity

[Rainyan]

In a perfect world, we should refactor for unit tests with comprehensive code coverage, at some future point.

But in the meanwhile, we ought to have a GH action runner (with a GH secrets var for a placeholder Discord bot token, set as NTBOT_SECRET_TOKEN), to verify that the bot launches properly without any errors. This would among other things, catch invalid StrictYAML schema errors.