[libc++][test] Don't use _LIBCPP macros.

[Mordante]

Summary

This was discovered by @ldionne.

libcxx/test/support/test_format_context.h

 #include "test_macros.h"
 
 #if TEST_STD_VER < 20
 #error "The format header requires at least C++20"
 #endif
 
 #include <format>
 
-#ifndef _LIBCPP_HAS_NO_LOCALIZATION
-#include <locale>
+#ifndef TEST_HAS_NO_LOCALIZATION
+#  include <locale>
 #endif
 
-#if defined(_LIBCPP_VERSION)
+#ifdef _LIBCPP_VERSION
 
 /** Creates a std::basic_format_context as-if the formatting function takes no locale. */
 template <class OutIt, class CharT>
 std::basic_format_context<OutIt, CharT> test_format_context_create(
     OutIt out_it,
     std::basic_format_args<std::basic_format_context<OutIt, CharT>> args) {
   return std::__format_context_create(std::move(out_it), args);
 }
 
-#ifndef _LIBCPP_HAS_NO_LOCALIZATION
+#  ifndef TEST_HAS_NO_LOCALIZATION
 /** Creates a std::basic_format_context as-if the formatting function takes locale. */
 template <class OutIt, class CharT>
 std::basic_format_context<OutIt, CharT> test_format_context_create(
     OutIt out_it,
     std::basic_format_args<std::basic_format_context<OutIt, CharT>> args,
     std::locale loc) {
   return std::__format_context_create(std::move(out_it), args, std::move(loc));
 }
-#endif
-#else
-#error                                                                         \
-    "Please create a vendor specific version of the test functions and file a review at https://reviews.llvm.org/"
-#endif
+#  endif // TEST_HAS_NO_LOCALIZATION
+#else    // _LIBCPP_VERSION
+#  error "Please create a vendor specific version of the test functions and file a review at https://reviews.llvm.org/"
+#endif // _LIBCPP_VERSION
 
 #endif // SUPPORT_TEST_FORMAT_CONTEXT_HPP