* 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.
* [Compression] Ensure we use the correct lonking flags for older versions. Fixes#4129.
Libcompresison was added after iOs 9.0, TvOS 9.0, Mac 10.11 and Watch
2.0. We want to use weak in those older platforms.
Fixes issue https://github.com/xamarin/xamarin-macios/issues/4129
Rolf Bjarne Kvinge
Manuel de la Pena