test: favor === over == in vm test

PR-URL: #8191
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: jbergstroem - Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/parallel/test-vm-sigint-existing-handler.js

@@ -43,7 +43,7 @@ if (process.argv[2] === 'child') {
 
   process.on('SIGINT', common.mustCall(() => {
     // Handler attached _after_ execution.
-    if (afterHandlerCalled++ == 0) {
+    if (afterHandlerCalled++ === 0) {
       // The first time it just bounces back to check that the `once()`
       // handler is not called the second time.
       process.kill(parent, 'SIGUSR2');