Only define wxUSE_XTEST in Unix-specific headers

This symbol is not needed and doesn't make sense in wxMSW.

include/wx/unix/chkconf.h

@@ -41,3 +41,11 @@
 #       endif
 #   endif
 #endif /* wxUSE_GSTREAMER */
+
+#ifndef wxUSE_XTEST
+#   ifdef wxABORT_ON_CONFIG_ERROR
+#       error "wxUSE_XTEST must be defined, please read comment near the top of this file."
+#   else
+#       define wxUSE_XTEST 0
+#   endif
+#endif /* !defined(wxUSE_XTEST) */