test: refactor test-stream-unshift-read-race

* add RegExp as second argument to assert.throws()
* replace process.on('exit', ...) boolean checks with common.mustCall()
* assert.equal() -> assert.strictEqual()
* add 1 ms duration as second argument to setTimeout()
* var -> const

PR-URL: https://github.com/nodejs/node/pull/10532


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>