* 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.
* [tests] Use Assert.Throws instead of the ExpectedException attribute.
The ExpectedExceptionAttribute doesn't exist in newer versions of NUnit[Lite] anymore.
* [monotouch-test] Rework some CaptiveNetwork tests.
They need permissions, and entitlements, and something else I couldn't figure
out, so just assert they're returning null instead.
* 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