test: refactor test-stdin-script-child

- var -> const where possible
- assert.equal -> assert.strictEqual
- passed the setTimeout function a second parameter for readability
- used assert.strictEqual for assert(!c) as it is expected to be 0 and
  not some other value

PR-URL: https://github.com/nodejs/node/pull/10321


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>