[tests] Fix ignoring members based on OSPlatformAttributes.

We were ignorning members based on whether a member had
ObsoletedOSPlatformAttribute/UnavailableOSPlatformAttribute, not taking into
account that the attribute might not apply to the current platform.

So implement logic to only take into account
[Obsoleted|Unavailable]OSPlatformAttributes for the current platform.

tests/common/TestRuntime.cs

@@ -12,6 +12,7 @@
 using System.Runtime.InteropServices;
 using System.Reflection;
 using System.Reflection.Emit;
+using System.Runtime.Versioning;
 
 using AVFoundation;
 using CoreBluetooth;
@@ -116,6 +117,59 @@ public static Version OSXVersion {
 	}
 #endif
 
+	public static ApplePlatform CurrentPlatform {
+		get {
+#if __MACCATALYST__
+			return ApplePlatform.MacCatalyst;
+#elif __IOS__
+			return ApplePlatform.iOS;
+#elif __TVOS__
+			return ApplePlatform.TVOS;
+#elif __MACOS__
+			return ApplePlatform.MacOSX;
+#elif __WATCHOS__
+			return ApplePlatform.WatchOS;
+#else
+#error Unknown platform
+#endif
+		}
+	}
+
+	// This returns the string for the platform as used in the OSPlatformAttribute's PlatformName property.
+	public static string GetOSPlatformName (ApplePlatform platform)
+	{
+		switch (platform) {
+		case ApplePlatform.iOS:
+			return "ios";
+		case ApplePlatform.MacOSX:
+			return "macos";
+		case ApplePlatform.TVOS:
+			return "tvos";
+		case ApplePlatform.MacCatalyst:
+			return "maccatalyst";
+		default:
+			throw new Exception ($"Unknown platform: {platform}");
+		}
+	}
+
+	public static bool HasOSPlatformAttributeForCurrentPlatform<T> (ICustomAttributeProvider provider) where T: OSPlatformAttribute
+	{
+		return HasOSPlatformAttribute<T> (provider, CurrentPlatform);
+	}
+
+	public static bool HasOSPlatformAttribute<T> (ICustomAttributeProvider provider, ApplePlatform platform) where T: OSPlatformAttribute
+	{
+		var attribs = provider.GetCustomAttributes (false);
+		var platformName = GetOSPlatformName (platform);
+		foreach (var attrib in attribs) {
+			if (attrib is T platformAttribute) {
+				if (platformAttribute.PlatformName.StartsWith (platformName, StringComparison.OrdinalIgnoreCase))
+					return true;
+			}
+		}
+		return false;
+	}
+
 	public static Version GetSDKVersion ()
 	{
 		var v = dyld_get_program_sdk_version ();

tests/introspection/ApiBaseTest.cs

@@ -196,7 +196,16 @@ protected bool SkipDueToRejectedTypes (Type type)
 		public bool MemberHasObsolete (MemberInfo member)
 		{
 #if NET
-			return member.GetCustomAttributes<ObsoletedOSPlatformAttribute> (false).Any ();
+			return TestRuntime.HasOSPlatformAttributeForCurrentPlatform<ObsoletedOSPlatformAttribute> (member);
+#else
+			return member.GetCustomAttribute<ObsoleteAttribute> () is not null;
+#endif
+		}
+
+		public bool MemberHasUnsupported (MemberInfo member)
+		{
+#if NET
+			return TestRuntime.HasOSPlatformAttributeForCurrentPlatform<UnsupportedOSPlatformAttribute> (member);
 #else
 			return member.GetCustomAttribute<ObsoleteAttribute> () is not null;
 #endif

tests/introspection/ApiSignatureTest.cs

@@ -22,6 +22,7 @@
 using System;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
 using System.Reflection;
 using System.Text;
 using NUnit.Framework;
@@ -864,10 +865,15 @@ static bool IsDiscouraged (MemberInfo mi)
 				case "AdviceAttribute":
 				case "ObsoletedAttribute":
 				case "DeprecatedAttribute":
-				case "UnsupportedOSPlatformAttribute":
 					return true;
 				}
 			}
+
+#if NET
+			if (TestRuntime.HasOSPlatformAttributeForCurrentPlatform<UnsupportedOSPlatformAttribute> (mi))
+				return true;
+#endif
+
 			return false;
 		}