Follow up to https://github.com/xamarin/xamarin-macios/pull/10658
This includes a `.ctor` chaining fix for the forwarder tool which
resulted in several errors like:
```
build/dotnet/maccatalyst/ref/Xamarin.iOS.cs(12640,17): error CS1729: 'UIScene' does not contain a constructor that takes 0 arguments
```
There's two different issues blocking ARKit and AddressBookUI that
will be solved in different PR.
When used with Xamarin.MacCatalyst we must still consider Xamarin.iOS.dll
as a platform assembly (and not user code) so the linker will remove more
code (that usual).
This is even more important as the stubs are unusable - so we do not want
to keep any extra pieces there since they would not work anyway.
* Fix system version checks to work properly on Mac Catalyst (which uses the macOS
version as its system version).
* Add the framework-specific defines to the build for monotouch-test.csproj (using
the generated response files), this way we can use them in the tests.
* Sprinkle conditionals in numerous places - I tried using either framework-specific
or XAMCORE_3_0 whenever that made since (instead of using Mac Catalyst as a condition).
* Updated a few tests to use non-deprecated API (because the deprecated API often
doesn't exist in Mac Catalyst).
* Also a few minor API fixes to make any corresponding tests compile.
Rolf Bjarne Kvinge
Sebastien Pouliot
Whitney Schmidt