test: improve test stream transform constructor

* new test for the error when a transform function is not specified
* use let instead of var
* use assert.strictEqual instead of assert.equal
* use arrow functions

PR-URL: https://github.com/nodejs/node/pull/10699


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>