Make the message on BLOWS_UP optional.

include/fuc2/testing.hpp

@@ -54,6 +54,6 @@ namespace fuc2 {
     }
   }
 
-  void BLOWS_UP(std::function<void()> cb, const std::string &message,
+  void BLOWS_UP(std::function<void()> cb, const std::string &message="",
       const std::source_location location = std::source_location::current());
 }