test: enforce strict mode for test-domain-crypto

The last change to this test landed before a nit about strict mode was
addressed, so this change addresses that.

PR-URL: https://github.com/nodejs/node/pull/6047
Refs: https://github.com/nodejs/node/pull/6017


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>