[tests] Use the correct corlib assembly name in linkall's AttributeTest.

Fixes this linkall test failure:

    LinkAll.Attributes.AttributeTest
        [FAIL] DebuggerTypeProxy_24203 :   proxy
            Expected: not null
            But was:  null
                at LinkAll.Attributes.AttributeTest.DebuggerTypeProxy_24203() in /Users/rolf/work/maccore/whatever/xamarin-macios/tests/linker/ios/link all/AttributeTest.cs:line 191

tests/linker/ios/link all/AttributeTest.cs

@@ -122,7 +122,11 @@ static public bool Check ()
 	public class AttributeTest {
 
 		// Good enough to fool linker to abort the tracking
+#if NET
+		static string mscorlib = "System.Private.CoreLib";
+#else
 		static string mscorlib = "mscorlib";
+#endif
 
 		[Test]
 		public void DebugAssemblyAttributes ()