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.

Refs: nodejs#6017

Author: Trott

test/parallel/test-domain-crypto.js

@@ -1,4 +1,6 @@
-/* eslint-disable strict, required-modules */
+/* eslint-disable required-modules */
+'use strict';
+
 try {
   var crypto = require('crypto');
 } catch (e) {