Fix harmless Clang warning about unused class member in a test.

The m_i field is only used in the currently commented out debugging messages,
suppress a warning about it being unused.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74521 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/thread/misc.cpp

@@ -168,6 +168,8 @@ public:
 
     virtual ExitCode Entry()
     {
+        wxUnusedVar(m_i);
+
         //wxPrintf(wxT("%s: Thread #%d (%ld) starting to wait for semaphore...\n"),
         //         wxDateTime::Now().FormatTime().c_str(), m_i, (long)GetId());