Tests: guard sweep_allocator::construct() against const types instead of const_cast

The const_cast added in the previous commit is rejected by
cppcoreguidelines-pro-type-const-cast (clang-tidy). Make construct() not
viable for const U instead, which is what libc++ probes for.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasEsrzHhSNouxSoEwqz
(cherry picked from commit de099fa607001b710862fd1bbecef3609c54cbc4)
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-allocator.cpp

@@ -14,6 +14,7 @@ using nlohmann::json;
 
 #include <functional>
 #include <limits>
+#include <type_traits>
 #include <valarray>
 #if JSON_HAS_RANGES
     #include <ranges>
@@ -1073,12 +1074,12 @@ struct sweep_allocator
         return (std::numeric_limits<std::size_t>::max)() / sizeof(T);
     }
 
-    // p may point to a const object when libc++ probes whether construct() is
+    // libc++ probes whether construct() is usable for const objects (when
-    // usable for copying, which static_cast<void*>(p) would not accept
+    // checking for trivial copies); this must not be viable then
-    template<class U, class... Args>
+    template<class U, class... Args, typename = typename std::enable_if<!std::is_const<U>::value>::type>
-    void construct(U* p, Args&& ... args) noexcept(noexcept(::new (const_cast<void*>(static_cast<const volatile void*>(p))) U(std::forward<Args>(args)...)))
+    void construct(U* p, Args&& ... args) noexcept(noexcept(::new (static_cast<void*>(p)) U(std::forward<Args>(args)...)))
     {
-        ::new (const_cast<void*>(static_cast<const volatile void*>(p))) U(std::forward<Args>(args)...);
+        ::new (static_cast<void*>(p)) U(std::forward<Args>(args)...);
     }
 
     template<class U>