doc: restrict the ES.Next features usage in tests

PR-URL: nodejs#11452
Refs: nodejs#11290
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: davidtaikocha

doc/guides/writing-tests.md

@@ -189,6 +189,21 @@ const assert = require('assert');
 const freelist = require('internal/freelist');
 ```
 
+### ES.Next features
+
+For performance considerations, we only use a selected subset of ES.Next
+features in JavaScript code in the `lib` directory. However, when writing
+tests, for the ease of backporting, it is encouraged to use those ES.Next
+features that can be used directly without a flag in [all maintained branches]
+(https://github.com/nodejs/lts), you can check [node.green](http://node.green)
+for all available features in each release.
+
+For example:
+
+* `let` and `const` over `var`
+* Template literals over string concatenation
+* Arrow functions when appropriate
+
 ## Naming Test Files
 
 Test files are named using kebab casing. The first component of the name is