tools,test: enable multiline-comment-style rule in tests

Use `//` for multline comments.

PR-URL: nodejs#35485
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/.eslintrc.yaml

@@ -5,11 +5,10 @@ env:
   es6: true
 
 rules:
-  # ECMAScript 6
-  # http://eslint.org/docs/rules/#ecmascript-6
   no-var: error
   prefer-const: error
   symbol-description: off
+  multiline-comment-style: ["error", "separate-lines"]
 
   no-restricted-syntax:
     # Config copied from .eslintrc.js