Skip System.Diagnostics.TextWriterTraceListenerTests.XmlWriterTraceListenerTests on iOS/tvOS (dotnet#61807)

This marks System.Diagnostics.TextWriterTraceListenerTests.XmlWriterTraceListenerTests withSkipOnPlatform attribute for iOS/tvOS as those tests try to create a process info, which throws PNSE after S.D.Process API's around libproc have been excluded in dotnet#61590.

Author: MaximLipnin

src/libraries/System.Diagnostics.TextWriterTraceListener/tests/XmlWriterTraceListenerTests.cs

@@ -12,6 +12,7 @@
 
 namespace System.Diagnostics.TextWriterTraceListenerTests
 {
+    [SkipOnPlatform(TestPlatforms.iOS | TestPlatforms.tvOS, "libproc is not supported on iOS/tvOS")]
     public class XmlWriterTraceListenerTests : FileCleanupTestBase
     {
         private readonly string _processName;