[libc++] [test] {cpo,niebloid}.compile.pass.cpp: Also test their constness.

This will detect if someone writes `inline auto cpo =` instead of
`inline constexpr auto cpo =`. I don't know how that'd be possible,
but it's easy to test, so let's test it.

libcxx/test/std/library/description/conventions/customization.point.object/cpo.compile.pass.cpp

@@ -13,13 +13,16 @@
 // [range.adaptor.object] "A range adaptor object is a customization point object..."
 
 #include <compare>
+#include <concepts>
 #include <iterator>
 #include <ranges>
+#include <type_traits>
 #include <utility>
 
 // Test for basic properties of C++20 16.3.3.3.6 [customization.point.object].
 template <class CPO, class... Args>
 constexpr bool test(CPO& o, Args&&...) {
+  static_assert(std::is_const_v<CPO>);
   static_assert(std::is_class_v<CPO>);
   static_assert(std::is_trivial_v<CPO>);
 

libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp

@@ -32,6 +32,7 @@
 
 template <class CPO, class... Args>
 constexpr bool test(CPO& o, Args&&...) {
+  static_assert(std::is_const_v<CPO>);
   static_assert(std::is_class_v<CPO>);
   static_assert(std::is_trivial_v<CPO>);