* [tests] Add a .NET/macOS version of monotouch-test.
I made a macOS version of monotouch-test instead of a .NET version of xammac tests,
so that one day we might have only one test suite for all our API tests.
* Add a project file for .NET/macOS
* Fix some code to handle the fact that we're called 'monotouchtest' on macOS (but
only on .NET).
* Ignore exception marshalling tests, those aren't working yet.
* [xharness] Add support for .NET/macOS and add a macOS version of monotouch-test to our tests
* [dotnet-linker] Skip libSystem.Net.Security.Native and libSystem.Native when collecting native methods to preserve for .NET/macOS.
* 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.
* Fix many version checks to be based on Xcode version instead of iOS version.
* Added/fixed a few expected values according to platform version to match behavior in older macOS versions.
Rolf Bjarne Kvinge
Timothy Risi