test: cleanup stream tests

const and let instead var
assert.strictEqual instead assert.equal

PR-URL: https://github.com/nodejs/node/pull/8668


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>