* Add additional app extensions to the list of items we need to sign.
* Improve msbuild test for additional app extensions:
* Build for both device and simulator.
* Hopefully fix the signing problems that occurred on the bots last time we tried.
* Assert that both the container and extension are signed during the build when we build for device.
A 10-line fix with 3300 lines of tests...
Fixes https://github.com/xamarin/xamarin-macios/issues/15598.