test: enable mustCall() during child exit

PR-URL: #16915
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/parallel/test-process-raw-debug.js

@@ -50,10 +50,10 @@ function parent() {
     console.log('ok - got expected message');
   });
 
-  child.on('exit', function(c) {
+  child.on('exit', common.mustCall(function(c) {
     assert(!c);
     console.log('ok - child exited nicely');
-  });
+  }));
 }
 
 function child() {