test: cleanup parallel/test-file-write-stream2.js

replace ==, !=, some instances of var, and assert.equal()

PR-URL: https://github.com/nodejs/node/pull/8770


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>