test: refactor test-process-kill-null

Use `common.mustCall()` instead of `called` variable.

PR-URL: #16236
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-process-kill-null.js

@@ -1,27 +1,21 @@
 'use strict';
-require('../common');
+const { mustCall } = require('../common');
 const assert = require('assert');
-const spawn = require('child_process').spawn;
+const { spawn } = require('child_process');
 
 const cat = spawn('cat');
-let called;
 
 assert.ok(process.kill(cat.pid, 0));
 
-cat.on('exit', function() {
+cat.on('exit', mustCall(function() {
   assert.throws(function() {
     process.kill(cat.pid, 0);
   }, Error);
-});
+}));
 
-cat.stdout.on('data', function() {
-  called = true;
+cat.stdout.on('data', mustCall(function() {
   process.kill(cat.pid, 'SIGKILL');
-});
+}));
 
 // EPIPE when null sig fails
 cat.stdin.write('test');
-
-process.on('exit', function() {
-  assert.ok(called);
-});