[MacRunner] Add availability attribute to avoid a compiler warning. (#…

…107)

* [MacRunner] Add availability attribute to avoid a compiler warning.

Fixes this warning:

    xamarin-macios/tests/common/mac/MacMain.cs(24,26): warning CA1416: This call site is reachable on: 'macOS/OSX' 10.11 and later. 'MacRunner.MainAsync(IList<string>, bool, Action<int>, params Assembly[])' is only supported on: 'macOS/OSX' 12.0 and later.

* Augment attributes.

NUnitLite/TouchRunner/TouchRunner.cs

@@ -27,6 +27,7 @@
 using System.Linq;
 using System.Reflection;
 using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
@@ -61,6 +62,13 @@
 using SettingsDictionary = System.Collections.IDictionary;
 #endif
 
+#if NET
+[assembly: SupportedOSPlatform ("ios10.0")]
+[assembly: SupportedOSPlatform ("tvos10.0")]
+[assembly: SupportedOSPlatform ("macos10.14")]
+[assembly: SupportedOSPlatform ("maccatalyst13.0")]
+#endif
+
 namespace MonoTouch.NUnit.UI {
 	public abstract class BaseTouchRunner : ITestListener {
 		TestSuite suite = new TestSuite ("TestSuite");