tests: switch test runner to mocha

[msimerson]

• nodeunit was deprecated 6 years ago
• a few years back, I switched us to the somewhat maintained nodeunit-x
• the history: nodeunit -> expresso -> mocha
• I use mocha and node --test everywhere else
• I still prefer mocha (node --test has sharp edges) but I expect the future is inevitably node --test.
• this PR has 98% of the changes (using the built-in assert module) needed to use node --test.
  • The first showstopper I encountered was test/server.js doesn't exit. I found (with wtfnode) one setInterval that needed to be unref'ed. There's nothing still listening, so that one requires a deeper dive.
• test: install deps with --omit=optional (go faster)
• tests: lots of cleanups for es6 & brevity

Checklist:

• docs updated
• tests updated
• Changes updated