Make options optional, and they'll be set by an option parser later.

include/fuc2/testing.hpp

@@ -9,12 +9,12 @@ namespace fuc2 {
   using Case = std::pair<std::string, Func>;
 
   struct Options {
-    bool fail_fast;
+    bool fail_fast = false;
   };
 
   struct Set {
     std::string name;
-    Options options;
+    Options options{};
     std::vector<Case> tests;
     std::source_location location = std::source_location::current();
   };