Added test in controls gallery for issue #13258

[beeradmoore]

Description of Change

Adds test in the gallery project for replicating Issue #13258. This was created so I could fix the underlying issue. I was not able to find the underlying issue but hopefully someone else can use this to debug XF source and find the problem.

Issues Resolved

None

API Changes

None

Platforms Affected

None

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

Launch Xamarin.Forms.ControlGallery.iOS on an iOS device or simulator
Click open issues
Search for 13258
Click the PushModal button then click PopModal on the new page.
Observe the following output in the IDE.

Issue13258_ModalPage OnAppearing
Issue13258 OnDisappearing
Issue13258 OnAppearing
Issue13258_ModalPage OnDisappearing

This shows that OnAppearing and OnDissapearing is working as expected.
Click the PushModal with Background Color button then click PopModal on the new page.
Observer that the output is now missing the OnAppearing/OnDissapearing of the underlying page.

Issue13258_ModalPage OnAppearing
Issue13258_ModalPage OnDisappearing

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[jsuarezruiz]

Thanks for your contribution @beeradmoore!