test: refactor test-file-write-stream

Replace all `var` occurrences in test-file-write-stream.js with
`const` (where they are not being reassigned) and `let` (where they are
being reassigned).

Add strict comparison to the asserts and if statements:

  - Replace `assert.equal` with `assert.strictEqual` where:
    1. Result of `typeof` being compared to a string literal.
    2. Result of `fs.readFileSync` with UTF-8 encoding being compared to
       a string constant.

  - Replace `==` with `===` where integer values are being compared to
    integer literals.

Remove unnecessary very IIFE.

Use template literals.

PR-URL: https://github.com/nodejs/node/pull/8894


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>