test: changed function to arrow function

Convert callback functions that are anonymous
to arrow functions for better readability.

PR-URL: nodejs#33711
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>

test/parallel/test-util-promisify.js

@@ -143,9 +143,7 @@ const stat = promisify(fs.stat);
 
   o.fn = fn;
 
-  o.fn().then(common.mustCall(function(val) {
-    assert(val);
-  }));
+  o.fn().then(common.mustCall((val) => assert(val)));
 }
 
 {