test: add mustCall() to child-process test

Add common.mustCall() check to test-child-process-stdio-big-write-end.

PR-URL: #13605
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-child-process-stdio-big-write-end.js

@@ -1,5 +1,5 @@
 'use strict';
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 let bufsize = 0;
 
@@ -22,10 +22,10 @@ function parent() {
   child.stdout.on('data', function(c) {
     n += c;
   });
-  child.stdout.on('end', function() {
+  child.stdout.on('end', common.mustCall(function() {
     assert.strictEqual(+n, sent);
     console.log('ok');
-  });
+  }));
 
   // Write until the buffer fills up.
   let buf;