Allow predefining wxTEST_DIALOG_HOOK_CLASS before wx/testing.h inclusion.

Although it already was (and remains) possible to #undef and re-#define
wxTEST_DIALOG_HOOK_CLASS after including wx/testing.h, it should also be
possible to just define it globally in the testing code before including
anything else and not have to bother with the #undef part.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78411 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/testing.h

@@ -338,7 +338,9 @@ private:
 
 // Redefining this value makes it possible to customize the hook class,
 // including e.g. its error reporting.
-#define wxTEST_DIALOG_HOOK_CLASS wxTestingModalHook
+#ifndef wxTEST_DIALOG_HOOK_CLASS
+    #define wxTEST_DIALOG_HOOK_CLASS wxTestingModalHook
+#endif
 
 #define WX_TEST_IMPL_ADD_EXPECTATION(pos, expect)                              \
     const wxModalExpectation& wx_exp##pos = expect;                            \