stdio-common: Add tests for formatted vfprintf output specifiers

Wire vfprintf into test infrastructure for formatted printf output
specifiers.

Reviewed-by: DJ Delorie <dj@redhat.com>

stdio-common/tst-printf-format-vf.h

@@ -0,0 +1,34 @@
+/* Test feature wrapper for formatted 'vfprintf' output.
+   Copyright (C) 2024 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 <stdarg.h>
+#include <stdio.h>
+
+static int
+printf_under_test (const char *restrict fmt, ...)
+{
+  va_list ap;
+  int result;
+
+  va_start (ap, fmt);
+  result = vfprintf (stdout, fmt, ap);
+  va_end (ap);
+  if (result < 0)
+    perror ("vfprintf");
+  return result;
+}