replace nodeunit with mocha

[msimerson]

Fixes #

➜  haraka-plugin-rspamd git:(master) ✗ npm install 
audited 443 packages in 4.17s

found 11 vulnerabilities (4 moderate, 6 high, 1 critical)
  run `npm audit fix` to fix them, or `npm audit` for details

Checklist:

• tests updated
• Changes.md updated