test: add tests for eslint rules

This adds tests for the custom eslint rules in this repository, using
the `RuleTester` test utility bundled with eslint.

PR-URL: https://github.com/nodejs/node/pull/16138
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-eslint-require-buffer.js

@@ -0,0 +1,26 @@
+'use strict';
+
+require('../common');
+
+const RuleTester = require('../../tools/eslint').RuleTester;
+const rule = require('../../tools/eslint-rules/require-buffer');
+const ruleTester = new RuleTester({
+  parserOptions: { ecmaVersion: 6 },
+  env: { node: true }
+});
+
+const message = "Use const Buffer = require('buffer').Buffer; " +
+                'at the beginning of this file';
+
+ruleTester.run('require-buffer', rule, {
+  valid: [
+    'foo',
+    'const Buffer = require("Buffer"); Buffer;'
+  ],
+  invalid: [
+    {
+      code: 'Buffer;',
+      errors: [{ message }]
+    }
+  ]
+});