Fix custom allocator test build failures (C++20)

Allocator tests fail to compile in C++20 mode with clang+MS STL due
to missing copy constructors.

test/src/unit-allocator.cpp

@@ -39,6 +39,8 @@ namespace
 template<class T>
 struct bad_allocator : std::allocator<T>
 {
+    using std::allocator<T>::allocator;
+
     template<class... Args>
     void construct(T* /*unused*/, Args&& ... /*unused*/)
     {

test/src/unit-regression2.cpp

@@ -271,7 +271,10 @@ std::string* sax_no_exception::error_string = nullptr;
 
 template<class T>
 class my_allocator : public std::allocator<T>
-{};
+{
+  public:
+    using std::allocator<T>::allocator;
+};
 
 /////////////////////////////////////////////////////////////////////
 // for #3077