Convert tests to Node test

[mattbishop]

We are using Mocha now, but it might be fine to use node's new built-in test framework instead.

[evert]

I wanted to see if this was a really quick fix, but it looks like oclifs own @oclif/test expects mocha:

https://oclif.io/docs/testing

So we either need to not use @oclif/test or stick to mocha for now.

[mattbishop]

Okay.