tools, test: require const/let in test

PR-URL: https://github.com/nodejs/node/pull/10685
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

test/.eslintrc

@@ -1,7 +1,13 @@
 ## Test-specific linter rules
 
 rules:
-  ## common module is mandatory in tests
-  required-modules: [2, common]
+  # ECMAScript 6
+  # http://eslint.org/docs/rules/#ecmascript-6
+  no-var: 2
+  prefer-const: 2
+
+  # Custom rules in tools/eslint-rules
   prefer-assert-iferror: 2
   prefer-assert-methods: 2
+  ## common module is mandatory in tests
+  required-modules: [2, common]