Smoketest the macOS package

[jdm]

After packaging when we have a DMG, we should mount that package and run Servo against a page that closes instantly and verify that it runs normally.

$ hdiutil attach target/debug/servo-tech-demo.dmg
$ cd /Volumes/Servo/Servo.app/Contents/MacOS
$ ./servo 'data:text/html,<script>window.onload = () => { console.log("success"); window.close(); };</script>'
$ hdiutil detach /Volumes/Servo

and look for both success and a return code of 0.

This would prevent bustage like #27318 in the future.