libstdc++: Add missing tests for std::basic_filebuf::native_handle()

I forgot to 'git add' these files in the commit that added the new
member function to basic_filebuf.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_filebuf/native_handle/char/1.cc: New test.
	* testsuite/27_io/basic_filebuf/native_handle/wchar_t/1.cc: New test.

libstdc++-v3/testsuite/27_io/basic_filebuf/native_handle/char/1.cc

+// { dg-options "-fno-inline" }
+// { dg-do run { target c++26 } }
+// { dg-additional-files "filebuf_members-1.txt" }
+
+#include <fstream>
+
+#ifndef __cpp_lib_fstream_native_handle
+# error "Feature-test macro for fstream_native_handle missing in <fstream>"
+#elif __cpp_lib_fstream_native_handle != 202306L
+# error "Feature-test macro for fstream_native_handle has wrong value in <fstream>"
+#endif
+
+using type = std::basic_filebuf<char>::native_handle_type;
+
+#include <cstdio> // std::fclose(FILE*)
+#if __has_include(<unistd.h>)
+# include <unistd.h> // close(int)
+#endif
+#if __has_include(<handleapi.h>)
+# include <handleapi.h> // CloseHandle(HANDLE)
+#endif
+
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::filebuf f;
+  f.open("filebuf_members-1.txt", std::ios::in);
+  type handle = f.native_handle();
+
+  auto native_close = []<typename HandleT>(HandleT handle) {
+    if constexpr (std::is_same_v<HandleT, std::FILE*>)
+      std::fclose(handle); // --enable-cstdio=stdio_pure
+#if __has_include(<unistd.h>)
+    else if constexpr (std::is_same_v<HandleT, int>)
+      ::close(handle); // POSIX
+#endif
+#if __has_include(<handleapi.h>)
+    else if constexpr (std::is_same_v<HandleT, void*>)
+      ::CloseHandle(handle); // Windows
+#endif
+    else
+      VERIFY( false );
+  };
+  native_close(handle);
+
+  try
+  {
+    f.sgetc();
+    VERIFY( false );
+  }
+  catch (const std::ios::failure&)
+  {
+  }
+}
+
+int main()
+{
+  test01();
+}

libstdc++-v3/testsuite/27_io/basic_filebuf/native_handle/wchar_t/1.cc

+// { dg-options "-fno-inline" }
+// { dg-do run { target c++26 } }
+// { dg-additional-files "filebuf_members-1.txt" }
+
+#include <fstream>
+
+using type = std::basic_filebuf<wchar_t>::native_handle_type;
+
+#include <cstdio> // std::fclose(FILE*)
+#if __has_include(<unistd.h>)
+# include <unistd.h> // close(int)
+#endif
+#if __has_include(<handleapi.h>)
+# include <handleapi.h> // CloseHandle(HANDLE)
+#endif
+
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::wfilebuf f;
+  f.open("filebuf_members-1.txt", std::wios::in);
+  type handle = f.native_handle();
+
+  auto native_close = []<typename HandleT>(HandleT handle) {
+    if constexpr (std::is_same_v<HandleT, std::FILE*>)
+      std::fclose(handle); // --enable-cstdio=stdio_pure
+#if __has_include(<unistd.h>)
+    else if constexpr (std::is_same_v<HandleT, int>)
+      ::close(handle); // POSIX
+#endif
+#if __has_include(<handleapi.h>)
+    else if constexpr (std::is_same_v<HandleT, void*>)
+      ::CloseHandle(handle); // Windows
+#endif
+    else
+      VERIFY( false );
+  };
+  native_close(handle);
+
+  try
+  {
+    f.sgetc();
+    VERIFY( false );
+  }
+  catch (const std::ios::failure&)
+  {
+  }
+}
+
+int main()
+{
+  test01();
+}