convert test modules to ES modules

[davidchambers]

This pull request converts all files in the test directory from CommonJS modules to ECMAScript modules.

test/internal/List and test/internal/Sum are used by doctests in the main module, which remains a CommonJS module, so they must also remain CommonJS modules for now. I first renamed them:

$ git mv test/internal/List.{js,cjs}
$ git mv test/internal/Sum.{js,cjs}

I then created wrapper modules for use within the test suite:

$ touch test/internal/{List,Sum}.mjs
$ git add test/internal/{List,Sum}.mjs

The only other non-trivial change involved using module.createRequire to keep the NODE_ENV tests working.