[libc++] Guard testing implementation details.

[Mordante]

Summary

The unit tests test some implementation details. As @Quuxplusone pointed
out in D96664 this should only be tested when the tests use libc++. This
addresses the issue for code already in main.

[ldionne]

LGTM, but it would be more consistent to replace both with LIBCPP_ASSERT.

libcxx/test/std/utilities/format/format.formatter/format.parse.ctx/check_arg_id.pass.cpp

41–42

We have LIBCPP_ASSERT.

[Mordante]

Addresses review comments.

[Mordante]

Thanks for the review! I updated the patch and will land it after the CI passes.

libcxx/test/std/utilities/format/format.formatter/format.parse.ctx/check_arg_id.pass.cpp

 
 void test_exception() {
   [] {
     std::format_parse_context context("", 1);
     context.next_arg_id();
     try {
       context.check_arg_id(0);
       assert(false);
     } catch (const std::format_error& e) {
-      assert(strcmp(e.what(), "Using manual argument numbering in automatic "
+      LIBCPP_ASSERT(strcmp(e.what(), "Using manual argument numbering in automatic "
                                      "argument numbering mode") == 0);
       return;
     }
     assert(false);
   }();
 
   auto test_arg = [](size_t num_args) {
     std::format_parse_context context("", num_args);
     // Out of bounds access is valid if !std::is_constant_evaluated()

libcxx/test/std/utilities/format/format.formatter/format.parse.ctx/next_arg_id.pass.cpp

 void test_exception() {
   std::format_parse_context context("", 1);
   context.check_arg_id(0);
 
   try {
     context.next_arg_id();
     assert(false);
   } catch (const std::format_error& e) {
-    assert(strcmp(e.what(), "Using automatic argument numbering in manual "
+    LIBCPP_ASSERT(strcmp(e.what(), "Using automatic argument numbering in manual "
                                    "argument numbering mode") == 0);
     return;
   }
   assert(false);
 }
 
 int main(int, char**) {
   test();
   test_exception();
   static_assert(test());
 
   return 0;
 }