stdio-common: Add tests for formatted sscanf input specifiers

Wire sscanf into test infrastructure for formatted scanf input
specifiers.

Reviewed-by: Joseph Myers <josmyers@redhat.com>

stdio-common/tst-scanf-format-ss.h

@@ -0,0 +1,73 @@
+/* Test feature wrapper for formatted 'scanf' input.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <support/support.h>
+
+static char *sscanf_buf;
+static size_t sscanf_buf_size;
+
+static void __attribute__ ((destructor))
+scanf_under_test_fini (void)
+{
+  free (sscanf_buf);
+}
+
+#define scanf_under_test(...)						\
+({									\
+  __label__ out;							\
+  size_t i = 0;								\
+  int result;								\
+  int ch;								\
+									\
+  do									\
+    {									\
+      ch = read_input ();						\
+      if (ch < 0)							\
+	{								\
+	  result = ch;							\
+	  goto out;							\
+	}								\
+      if (i == sscanf_buf_size)						\
+	{								\
+	  sscanf_buf_size += SIZE_CHUNK;				\
+	  /* Add an extra byte for the terminating null character.  */	\
+	  sscanf_buf = xrealloc (sscanf_buf, sscanf_buf_size + 1);	\
+	}								\
+      sscanf_buf[i++] = ch;						\
+    }									\
+  while (ch != ':');							\
+  sscanf_buf[i++] = '\0';						\
+									\
+  ch = ungetc (ch, stdin);						\
+  if (ch == EOF)							\
+    {									\
+      result = INPUT_ERROR;						\
+      goto out;								\
+    }									\
+									\
+  result = sscanf (sscanf_buf, __VA_ARGS__);				\
+  if (result == EOF)							\
+    result = INPUT_EOF;							\
+									\
+out:									\
+  result;								\
+})