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/testprec.h

@@ -4,6 +4,10 @@
 #include "wx/wxprec.h"
 #include "wx/stopwatch.h"
 #include "wx/evtloop.h"
+
+// This needs to be included before catch.hpp to be taken into account.
+#include "testdate.h"
+
 #include "wx/catch_cppunit.h"
 
 // Custom test macro that is only defined when wxUIActionSimulator is available