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/testdate.h

@@ -11,6 +11,8 @@
 
 #include "wx/datetime.h"
 
+#include <ostream>
+
 // need this to be able to use CPPUNIT_ASSERT_EQUAL with wxDateTime objects
 inline std::ostream& operator<<(std::ostream& ostr, const wxDateTime& dt)
 {