[libc++][format] Improves the handle test.

[Mordante]

Summary

A formatter using a handle only needs to satisfy the BasicFormatter
requirements. The current test allowed more than that minimum. Changed
it to the minimum to make sure it works.

This was due to a post-commit review comment of @vitaut in D121530.

[vitaut]

LGTM

libcxx/test/std/utilities/format/format.functions/format_tests.h

 // A user-defined type used to test the handle formatter.
 enum class status : uint16_t { foo = 0xAAAA, bar = 0x5555, foobar = 0xAA55 };
 
 // The formatter for a user-defined type used to test the handle formatter.
 template <class CharT>
 struct std::formatter<status, CharT> {
   int type = 0;
 
-  constexpr auto parse(auto& parse_ctx) -> decltype(parse_ctx.begin()) {
+  constexpr auto parse(basic_format_parse_context<CharT>& parse_ctx) -> decltype(parse_ctx.begin()) {
     auto begin = parse_ctx.begin();
     auto end = parse_ctx.end();
     if (begin == end)
       return begin;
 
     switch (*begin) {
     case CharT('x'):
       break;
constexpr auto parse(basic_format_parse_context<CharT>& parse_ctx) -> decltype(parse_ctx.begin()) {
 
     ++begin;
     if (begin != end && *begin != CharT('}'))
       throw_format_error("The format-spec should consume the input or end with a '}'");
 
     return begin;
   }
 
-  auto format(status s, auto& ctx) -> decltype(ctx.out()) {
+  template <class Out>
+  auto format(status s, basic_format_context<Out, CharT>& ctx) -> decltype(ctx.out()) {
     const char* names[] = {"foo", "bar", "foobar"};
     char buffer[6];
     const char* begin;
     const char* end;
     switch (type) {
     case 0:
       begin = buffer;
       buffer[0] = '0';