tools, test: require const/let in test

Backport-PR-URL: #11775
PR-URL: #10685
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

Author: gibfahn

test/.eslintrc.yaml

@@ -1,6 +1,11 @@
 ## Test-specific linter rules
 
 rules:
+  # ECMAScript 6
+  # http://eslint.org/docs/rules/#ecmascript-6
+  no-var: 2
+  prefer-const: 2
+
   ## common module is mandatory in tests
   required-modules: [2, common]
   prefer-assert-methods: 2