test: improve test-stream2-large-read-stall

* use const instead of var
* use assert.strictEqual instead of assert.equal
* use  common.mustCall instead of process.on( 'exit', fn )

PR-URL: https://github.com/nodejs/node/pull/10725


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>