You can validate web app changes in a staging deployment slot before swapping it with the production slot. Deploying an app to a slot first and swapping it into production makes sure that all instances of the slot are warmed up before being swapped into production. This eliminates downtime when you deploy your app. The traffic redirection is seamless, and no requests are dropped because of swap operations. You can automate this entire workflow by configuring auto swap when pre-swap validation isn't needed.
After the swap you can deploy the App1 update to webapp1-test, and then test the update. If the changes swapped into the production slot aren't as per your expectation then you can perform the same swap immediately to get your "last known good site" back.
Reference:
https://docs.microsoft.com/en-us/azure/app-service/deploy-staging-slots