[libc++][test] Improves handle formatter.

[Mordante]

Summary

Before it only accepted one output iterator type. Now it accepts all
output iterator types as required by BasicFormatter.

libcxx/test/std/utilities/format/format.formatter/format.formatter.spec/formatter.handle.pass.cpp

 
 #include "test_macros.h"
 
 enum class color { black, red, gold };
 const char* color_names[] = {"black", "red", "gold"};
 
 template <>
 struct std::formatter<color> : std::formatter<const char*> {
-  auto format(color c, format_context& ctx) {
-    return formatter<const char*>::format(color_names[static_cast<int>(c)], ctx);
-  }
+  auto format(color c, auto& ctx) { return formatter<const char*>::format(color_names[static_cast<int>(c)], ctx); }
 };
 
 void test(std::string expected, std::string_view fmt, color arg) {
   auto parse_ctx = std::format_parse_context(fmt);
   std::formatter<color, char> formatter;
   static_assert(std::semiregular<decltype(formatter)>);
 
   auto it = formatter.parse(parse_ctx);