Because `esm` does not support legacy versions of Node.js, at least not
below Node.js 6 as of writing this, we have to avoid using that whenever
tests are running on legacy versions.

But now that the source code (`mustache.js`) is written in ESM syntax,
how on earth are we going to run tests on these legacy versions of
Node.js? We gotta run the build step first, so that we end up with a
`mustache.js` file in CJS, or strictly speaking it will be UMD.

That's kinda pain in the backside isn't it? Yes, but running tests on
legacy versions are not meant to be done locally, but rather in CI. That
means we can easily automate the flow of (1) building the source code
before (2) starting the test suite.

For our futureselves, if we want to stop running tests on legacy
versions of Node.js; the changes introduces in this commit, could be
removed completely.