test: refactor test-domain-multi

Replace assert.equal() with assert.strictEqual().

PR-URL: https://github.com/nodejs/node/pull/9963


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>