Move away from vows

[sonnyp]

We use vows for testing. The CLI doesn't support ES modules so I had to programmatically run the tests which result in poor output.

In addition it doesn't appear to be actively maintained.

xmpp.js uses ava so that's an option but it's quite slow and while a good fit for the async nature of xmpps.js - I'm not sure it makes sense for ltx.

https://github.com/talentlessguy/awesome-node-esm#testing for alternatives