test: check number of message events

When a no-op message event handler is used in a test, make it clear what
is expected by using `common.mustCall()` and `common.mustNotCall()`.

PR-URL: #13125
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <mhdawson@ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: Trott

test/parallel/test-child-process-fork-ref2.js

@@ -29,7 +29,7 @@ if (process.argv[2] === 'child') {
 
   setTimeout(function() {
     console.log('child -> will this keep it alive?');
-    process.on('message', common.noop);
+    process.on('message', common.mustNotCall());
   }, 400);
 
 } else {

test/sequential/test-child-process-pass-fd.js

@@ -45,7 +45,7 @@ if (process.argv[2] !== 'child') {
   // the only thing keeping this worker alive will be IPC. This is important,
   // because it means a worker with no parent will have no referenced handles,
   // thus no work to do, and will exit immediately, preventing process leaks.
-  process.on('message', common.noop);
+  process.on('message', common.mustCall());
 
   const server = net.createServer((c) => {
     process.once('message', function(msg) {