Add back wxUSE_XTEST definition to Xcode setup.h file

This was erroneously removed by e82c619402 but
it still needs to be defined here, even if it's not used under macOS, in order
to avoid errors about it being undefined in wx/unix/chkconf.h.

include/wx/osx/setup0.h

@@ -1439,6 +1439,10 @@
 // Compile wxUIActionSimulator class?
 #define wxUSE_UIACTIONSIMULATOR 1
 
+// This is only used under Unix, but needs to be defined here as it's checked
+// by wx/unix/chkconf.h.
+#define wxUSE_XTEST 0
+
 // ----------------------------------------------------------------------------
 // wxDC classes for various output formats
 // ----------------------------------------------------------------------------