test: test: refactor test-sync-fileread

change equal to strictEqual and var to const

PR-URL: https://github.com/nodejs/node/pull/9941


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>