Include testdate.h before catch.hpp in unit tests

This ensures that dates are printed out correctly if comparing them
fails.

It might be better to avoid always including this header, but this is
the simplest solution.

tests/datetime/datetimetest.cpp

@@ -24,8 +24,6 @@
 
 #include "wx/wxcrt.h"       // for wxStrstr()
 
-#include "testdate.h"
-
 // to test Today() meaningfully we must be able to change the system date which
 // is not usually the case, but if we're under Win32 we can try it -- define
 // the macro below to do it