Also:
* Use the right arm64 runtime identifier depending on the target platform.
* Don't add CoreCLR variations of Mac Catalyst tests.
* Exclude UISearchDisplayController from tests, since it isn't available in
Mac Catalyst anymore.
* 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.