test: cleanup parallel/test-fs-readfile-unlink.js

Changes var to const, != to !==, and assert.equal() to assert.strict
Equal()

PR-URL: https://github.com/nodejs/node/pull/8764


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