These changes are primarily aiming for having an acceptable developer
experience when working on changes locally and want to run the test
suite.

By using `esm` as a required package to be executed *before* the
individual test files are executed, `esm` allows CommonJS/`require()`
based code to use code written with ES Modules syntax.

In practise that means our existing CommonJS based test suite that is
`require()`ing the source code witten in ES Modules, is seamless and
totally transparent. It just works without any build or transpiling
pipeline like `babel` involved.

Caveat: the `esm` package only support Node.js 6.x and above.

That is more than okey, as we've got continous integration to verify
how our changes works on different versions of Node.js, browsers & deno.

Refs https://www.npmjs.com/package/esm