test,child_process: add tests for signalCode value

Prior to this change, none of the child_process tests checked the
signalCode property for a value other than null. Add a check to an
existing test.

PR-URL: #35327
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>

Author: Trott

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

@@ -32,8 +32,10 @@ cat.stderr.on('end', common.mustCall());
 cat.on('exit', common.mustCall((code, signal) => {
   assert.strictEqual(code, null);
   assert.strictEqual(signal, 'SIGTERM');
+  assert.strictEqual(cat.signalCode, 'SIGTERM');
 }));
 
+assert.strictEqual(cat.signalCode, null);
 assert.strictEqual(cat.killed, false);
 cat.kill();
 assert.strictEqual(cat.killed, true);