doc: internal/test/binding for testing

PR-URL: #38026
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/guides/writing-tests.md

@@ -287,6 +287,15 @@ const assert = require('assert');
 const freelist = require('internal/freelist');
 ```
 
+In specific scenarios it may be useful to get a hold of `primordials` or
+`internalBinding()`. You can do so using
+
+```console
+node --expose-internals -r internal/test/binding lib/fs.js
+```
+
+This only works if you preload `internal/test/binding` by command line flag.
+
 ### Assertions
 
 When writing assertions, prefer the strict versions: