test: apply strict mode in test-repl

Strict mode for the test will not automatically enable strict mode
in the REPL object. Enable strict mode in the test.

PR-URL: #11575
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

Author: Trott

test/parallel/test-repl.js

@@ -1,4 +1,6 @@
-/* eslint-disable max-len, strict */
+/* eslint-disable max-len */
+'use strict';
+
 const common = require('../common');
 const assert = require('assert');