[tests] Ignore a don't link test that calls MulticastDelegate.BeginIn…

…voke.

MulticastDelegate.BeginInvoke isn't supported in .NET.

Ref: dotnet/runtime#16312

Fixes this test failure:

    DontLink.DontLinkRegressionTests
    [FAIL] Bug5354 : System.PlatformNotSupportedException : Operation is not supported on this platform.
        at DontLink.DontLinkRegressionTests.Bug5354() in [...]/xamarin-macios/tests/linker/ios/dont link/DontLinkRegressionTests.cs:line 64
        at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

tests/linker/ios/dont link/DontLinkRegressionTests.cs

@@ -58,6 +58,9 @@ public void RemovedAttributes ()
 		}
 
 		[Test]
+#if NET
+		[Ignore ("MulticastDelegate.BeginInvoke isn't supported in .NET (https://github.com/dotnet/runtime/issues/16312)")]
+#endif
 		public void Bug5354 ()
 		{
 			Action<string> testAction = (string s) => { s.ToString (); };