test: refactor domain test

Use assert.strictEqual() instead of assert.equal().

PR-URL: https://github.com/nodejs/node/pull/10269


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>