* 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.
Largely related to somewhat recent frameworks/API additions to XM
ExternalAccessory was added to macOS 10.13 (Xcode9) and can't be run before that (including on the jenkins sierra bots). Also enable that test on tvOS (which added support in Xcode8)
Alex Soto
Rolf Bjarne Kvinge
Sebastien Pouliot
Timothy Risi