Use `promisify` or `new Promise` pattern.

[relu91]

In some tests, we use promisify where the other places we use the new Promise pattern. Personally, I found promisify less verbose and more explicit, but we have to choose a consistent pattern for the codebase.

Originally posted by @relu91 in https://github.com/eclipse/thingweb.node-wot/pull/579#discussion_r728727283