[libc++][test] Fix invalid const conversion in limited_allocator (#118189)

This patch fixes a const-qualification on the return type of a method
of `limited_allocator`, which is widely used for testing allocator-aware
containers.

Author: winner245

libcxx/test/support/test_allocator.h

@@ -477,7 +477,7 @@ class limited_allocator {
   TEST_CONSTEXPR_CXX20 pointer allocate(size_type n) { return handle_->template allocate<T>(n); }
   TEST_CONSTEXPR_CXX20 void deallocate(pointer p, size_type n) { handle_->template deallocate<T>(p, n); }
   TEST_CONSTEXPR size_type max_size() const { return N; }
-  TEST_CONSTEXPR BuffT* getHandle() const { return handle_.get(); }
+  TEST_CONSTEXPR const BuffT* getHandle() const { return handle_.get(); }
 };
 
 template <class T, class U, std::size_t N>