test: refactor test-signal-unregister

* var -> const
* assert.equal() -> assert.strictEqual()

PR-URL: https://github.com/nodejs/node/pull/9920


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>