How to sign old iOS version

I would love to be able to test my app against older iOS version but unfortunately I upgraded my iPhone to iOS 10...

When going in the downloads section of the Apple website I can't find older iOS images anymore, so I can't downgrade.

This is old news.

Is there a way to sign unsigned IPSW files so that they can be installed on an iPhone without bricking them?

This is so inconvenient. Can't really understand why they did this.