[libc++][test] Replace `_LIBCPP_STD_VER` with `TEST_STD_VER`

[philnik]

Do you need someone to commit it on your behalf?

[cpplearner]

In D130979#3693222, @philnik wrote:

Do you need someone to commit it on your behalf?

Yes, I do.

My name and email address: Tan S. B. <[e-mail]>

libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_copy.pass.cpp

struct Explicit {
   explicit Explicit(int x) : value(x) {}
 };
 
 struct Implicit {
   int value;
   Implicit(int x) : value(x) {}
 };
 
-#if _LIBCPP_STD_VER > 17
+#if TEST_STD_VER > 17
 constexpr bool alloc_copy_constructor_is_constexpr() {
   const std::tuple<int> t1 = 1;
   std::tuple<int> t2 = {std::allocator_arg, test_allocator<int>{}, t1};
   assert(std::get<0>(t2) == 1);
   return true;
 }
 #endif
 
int main(int, char**)
     {
         // Test that we can use a tag derived from allocator_arg_t
         struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
         DerivedFromAllocatorArgT derived;
         std::tuple<long> from(3l);
         std::tuple<long long> t0(derived, A1<int>(), from);
     }
 
-#if _LIBCPP_STD_VER > 17
+#if TEST_STD_VER > 17
     static_assert(alloc_copy_constructor_is_constexpr());
 #endif
     return 0;
 }

libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_convert_move.pass.cpp

struct Explicit {
   explicit Explicit(int x) : value(x) {}
 };
 
 struct Implicit {
   int value;
   Implicit(int x) : value(x) {}
 };
 
-#if _LIBCPP_STD_VER > 17
+#if TEST_STD_VER > 17
 constexpr bool alloc_move_constructor_is_constexpr() {
   std::tuple<int> t1 = 1;
   std::tuple<int> t2 = {std::allocator_arg, test_allocator<int>{}, std::move(t1)};
   assert(std::get<0>(t2) == 1);
   return true;
 }
 #endif
 
int main(int, char**)
     {
         // Test that we can use a tag derived from allocator_arg_t
         struct DerivedFromAllocatorArgT : std::allocator_arg_t { };
         DerivedFromAllocatorArgT derived;
         std::tuple<long> from(3l);
         std::tuple<long long> t0(derived, A1<int>(), std::move(from));
     }
 
-#if _LIBCPP_STD_VER > 17
+#if TEST_STD_VER > 17
     static_assert(alloc_move_constructor_is_constexpr());
 #endif
 
     return 0;
 }