Simplify testing tasks; make tests & linting optional on watch workflow

This will also move a few dependencies from the production dependencies to development dependencies side. However, I think this still should be done, because I think the next step is to move all the production dependencies to the development dependencies, and exclude the gulp build entirely from the production side (e.g. move the build to 'npm prepublish' side; as a result Docker builds etc. would speed up a lot).
That way we could skip Bluebird development dependency on production. But all (or at least most) of these deps will be moved into a development dependency soon after, so I would rather not react into this right now.