test: update test-stdout-to-file

* changed vars to const
* changed assert.equal to assert.strictEqual
* added a common.mustCall in the childProcess.exec callback
* replaced 2 console.log strings with template strings for readability
* had to break up line 9 because it was causing a line max length (80)
listing err

PR-URL: https://github.com/nodejs/node/pull/9939


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>