[NFC][libc++][test] Improves code reuse.

[Mordante]

Summary

This applies D140115 to the new tuple tests.

libcxx/test/std/utilities/format/format.tuple/format.functions.tests.h

 //===----------------------------------------------------------------------===//
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef TEST_STD_UTILITIES_FORMAT_FORMAT_TUPLE_FORMAT_TESTS_H
 #define TEST_STD_UTILITIES_FORMAT_FORMAT_TUPLE_FORMAT_TESTS_H
 
 #include <concepts>
 #include <format>
 
-#include "make_string.h"
-
-#define STR(S) MAKE_STRING(CharT, S)
-#define SV(S) MAKE_STRING_VIEW(CharT, S)
-
-template <class T>
-struct context {};
-
-template <>
-struct context<char> {
-  using type = std::format_context;
-};
-
-#ifndef TEST_HAS_NO_WIDE_CHARACTERS
-template <>
-struct context<wchar_t> {
-  using type = std::wformat_context;
-};
-#endif
-
-template <class T>
-using context_t = typename context<T>::type;
+#include "format.functions.common.h"
 
 enum class color { black, red, gold };
 
 template <class CharT>
 struct std::formatter<color, CharT> : std::formatter<basic_string_view<CharT>, CharT> {
   static constexpr basic_string_view<CharT> color_names[] = {SV("black"), SV("red"), SV("gold")};
   auto format(color c, auto& ctx) const {
     return formatter<basic_string_view<CharT>, CharT>::format(color_names[static_cast<int>(c)], ctx);

libcxx/test/support/format.functions.common.h

 //===----------------------------------------------------------------------===//
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef TEST_SUPPORT_FORMAT_FUNCTIONS_COMMON_H
 #define TEST_SUPPORT_FORMAT_FUNCTIONS_COMMON_H
 
 // Contains the common part of the formatter tests for different papers.
 
+#include <algorithm>
+#include <charconv>
 #include <format>
 
 #include "make_string.h"
 
 #define STR(S) MAKE_STRING(CharT, S)
 #define SV(S) MAKE_STRING_VIEW(CharT, S)
 #define CSTR(S) MAKE_CSTRING(CharT, S)