test: show actual error in next-tick-when-exiting

On process exit if some assertion error occurs value of
`process._exiting` was hidden, this fix will show the actual
error message with value.

PR-URL: #20956
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

test/parallel/test-next-tick-when-exiting.js

@@ -1,14 +1,14 @@
 'use strict';
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 
 process.on('exit', () => {
-  assert.strictEqual(process._exiting, true, 'process._exiting was not set!');
+  assert.strictEqual(process._exiting, true);
 
-  process.nextTick(() => {
-    assert.fail('process is exiting, should not be called.');
-  });
+  process.nextTick(
+    common.mustNotCall('process is exiting, should not be called')
+  );
 });
 
 process.exit();