use node: prefix for requires

mostly because code is being autogenerated by all the AI stuff using
this prefix. it's also used in the stack trace.

src/test/backupformat-test.js

@@ -10,9 +10,9 @@ const common = require('./common.js'),
     DataLayout = require('../datalayout.js'),
     EnsureFileSizeStream = require('../backupformat/tgz.js')._EnsureFileSizeStream,
     expect = require('expect.js'),
-    fs = require('fs'),
-    os = require('os'),
-    path = require('path'),
+    fs = require('node:fs'),
+    os = require('node:os'),
+    path = require('node:path'),
     rsync = require('../backupformat/rsync.js'),
     safe = require('safetydance'),
     stream = require('node:stream/promises');